NOTICE (18 Dec 99): See the file README.TXT!
How It Works -- Master Boot Record
Version 1b
by Hale Landis <hlandis@ibm.net>
MASTER BOOT RECORD
This article is a disassembly of a Master Boot Record (MBR). The
MBR is the sector at cylinder 0, head 0, sector 1 of a hard disk.
An MBR is created by the FDISK program. The FDISK program of all
operating systems must create a functionally similar MBR. The MBR
is first of what could be many partition sectors, each one
containing a four entry partition table.
At the completion of your system's Power On Self Test (POST), INT
19 is called. Usually INT 19 tries to read a boot sector from
the first floppy drive. If a boot sector is found on the floppy
disk, the that boot sector is read into memory at location
0000:7C00 and INT 19 jumps to memory location 0000:7C00.
However, if no boot sector is found on the first floppy drive,
INT 19 tries to read the MBR from the first hard drive. If an
MBR is found it is read into memory at location 0000:7c00 and INT
19 jumps to memory location 0000:7c00. The small program in the
MBR will attempt to locate an active (bootable) partition in its
partition table. If such a partition is found, the boot sector
of that partition is read into memory at location 0000:7C00 and
the MBR program jumps to memory location 0000:7C00. Each
operating system has its own boot sector format. The small
program in the boot sector must locate the first part of the
operating system's kernel loader program (or perhaps the kernel
itself or perhaps a "boot manager program") and read that into
memory.
INT 19 is also called when the CTRL-ALT-DEL keys are used. On
most systems, CTRL-ALT-DEL causes an short version of the POST to
be executed before INT 19 is called.
=====
Where stuff is:
The MBR program code starts at offset 0000.
The MBR messages start at offset 008b.
The partition table starts at offset 01be.
The signature is at offset 01fe.
Here is a summary of what this thing does:
If an active partition is found, that partition's boot record
is read into 0000:7c00 and the MBR code jumps to 0000:7c00
with SI pointing to the partition table entry that describes
the partition being booted. The boot record program uses this
data to determine the drive being booted from and the location
of the partition on the disk.
If no active partition table enty is found, ROM BASIC is
entered via INT 18. All other errors cause a system hang, see
label HANG.
NOTES (VERY IMPORTANT):
1) The first byte of an active partition table entry is 80.
This byte is loaded into the DL register before INT 13 is
called to read the boot sector. When INT 13 is called, DL is
the BIOS device number. Because of this, the boot sector read
by this MBR program can only be read from BIOS device number
80 (the first hard disk). This is one of the reasons why it
is usually not possible to boot from any other hard disk.
2) The MBR program uses the CHS based INT 13H AH=02H call to
read the boot sector of the active partition. The location of
the active partition's boot sector is in the partition table
entry in CHS format. If the drive is >528MB, this CHS must be
a translated CHS (or L-CHS, see my BIOS TYPES document).
No addresses in LBA form are used (another reason why LBA
doesn't solve the >528MB problem).
=====
Here is the entire MBR record (hex dump and ascii).
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 fa33c08e d0bc007c 8bf45007 501ffbfc *.3.....|..P.P...*
000010 bf0006b9 0001f2a5 ea1d0600 00bebe07 *................*
000020 b304803c 80740e80 3c00751c 83c610fe *...<.t..<.u.....*
000030 cb75efcd 188b148b 4c028bee 83c610fe *.u......L.......*
000040 cb741a80 3c0074f4 be8b06ac 3c00740b *.t..<.t.....<.t.*
000050 56bb0700 b40ecd10 5eebf0eb febf0500 *V.......^.......*
000060 bb007cb8 010257cd 135f730c 33c0cd13 *..|...W.._s.3...*
000070 4f75edbe a306ebd3 bec206bf fe7d813d *Ou...........}.=*
000080 55aa75c7 8bf5ea00 7c000049 6e76616c *U.u.....|..Inval*
000090 69642070 61727469 74696f6e 20746162 *id partition tab*
0000a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000d0 696e6720 73797374 656d0000 00000000 *ing system......*
0000e0 00000000 00000000 00000000 00000000 *................*
0000f0 TO 0001af SAME AS ABOVE
0001b0 00000000 00000000 00000000 00008001 *................*
0001c0 0100060d fef83e00 00000678 0d000000 *......>....x....*
0001d0 00000000 00000000 00000000 00000000 *................*
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
=====
Here is the disassembly of the MBR...
This sector is initially loaded into memory at 0000:7c00 but
it immediately relocates itself to 0000:0600.
BEGIN: NOW AT 0000:7C00, RELOCATE
0000:7C00 FA CLI disable int's
0000:7C01 33C0 XOR AX,AX set stack seg to 0000
0000:7C03 8ED0 MOV SS,AX
0000:7C05 BC007C MOV SP,7C00 set stack ptr to 7c00
0000:7C08 8BF4 MOV SI,SP SI now 7c00
0000:7C0A 50 PUSH AX
0000:7C0B 07 POP ES ES now 0000:7c00
0000:7C0C 50 PUSH AX
0000:7C0D 1F POP DS DS now 0000:7c00
0000:7C0E FB STI allow int's
0000:7C0F FC CLD clear direction
0000:7C10 BF0006 MOV DI,0600 DI now 0600
0000:7C13 B90001 MOV CX,0100 move 256 words (512 bytes)
0000:7C16 F2 REPNZ move MBR from 0000:7c00
0000:7C17 A5 MOVSW to 0000:0600
0000:7C18 EA1D060000 JMP 0000:061D jmp to NEW_LOCATION
NEW_LOCATION: NOW AT 0000:0600
0000:061D BEBE07 MOV SI,07BE point to first table entry
0000:0620 B304 MOV BL,04 there are 4 table entries
SEARCH_LOOP1: SEARCH FOR AN ACTIVE ENTRY
0000:0622 803C80 CMP BYTE PTR [SI],80 is this the active entry?
0000:0625 740E JZ FOUND_ACTIVE yes
0000:0627 803C00 CMP BYTE PTR [SI],00 is this an inactive entry?
0000:062A 751C JNZ NOT_ACTIVE no
0000:062C 83C610 ADD SI,+10 incr table ptr by 16
0000:062F FECB DEC BL decr count
0000:0631 75EF JNZ SEARCH_LOOP1 jmp if not end of table
0000:0633 CD18 INT 18 GO TO ROM BASIC
FOUND_ACTIVE: FOUND THE ACTIVE ENTRY
0000:0635 8B14 MOV DX,[SI] set DH/DL for INT 13 call
0000:0637 8B4C02 MOV CX,[SI+02] set CH/CL for INT 13 call
0000:063A 8BEE MOV BP,SI save table ptr
SEARCH_LOOP2: MAKE SURE ONLY ONE ACTIVE ENTRY
0000:063C 83C610 ADD SI,+10 incr table ptr by 16
0000:063F FECB DEC BL decr count
0000:0641 741A JZ READ_BOOT jmp if end of table
0000:0643 803C00 CMP BYTE PTR [SI],00 is this an inactive entry?
0000:0646 74F4 JZ SEARCH_LOOP2 yes
NOT_ACTIVE: MORE THAN ONE ACTIVE ENTRY FOUND
0000:0648 BE8B06 MOV SI,068B display "Invld prttn tbl"
DISPLAY_MSG: DISPLAY MESSAGE LOOP
0000:064B AC LODSB get char of message
0000:064C 3C00 CMP AL,00 end of message
0000:064E 740B JZ HANG yes
0000:0650 56 PUSH SI save SI
0000:0651 BB0700 MOV BX,0007 screen attributes
0000:0654 B40E MOV AH,0E output 1 char of message
0000:0656 CD10 INT 10 to the display
0000:0658 5E POP SI restore SI
0000:0659 EBF0 JMP DISPLAY_MSG do it again
HANG: HANG THE SYSTEM LOOP
0000:065B EBFE JMP HANG sit and stay!
READ_BOOT: READ ACTIVE PARITION BOOT RECORD
0000:065D BF0500 MOV DI,0005 INT 13 retry count
INT13RTRY: INT 13 RETRY LOOP
0000:0660 BB007C MOV BX,7C00
0000:0663 B80102 MOV AX,0201 read 1 sector
0000:0666 57 PUSH DI save DI
0000:0667 CD13 INT 13 read sector into 0000:7c00
0000:0669 5F POP DI restore DI
0000:066A 730C JNB INT13OK jmp if no INT 13
0000:066C 33C0 XOR AX,AX call INT 13 and
0000:066E CD13 INT 13 do disk reset
0000:0670 4F DEC DI decr DI
0000:0671 75ED JNZ INT13RTRY if not zero, try again
0000:0673 BEA306 MOV SI,06A3 display "Errr ldng systm"
0000:0676 EBD3 JMP DISPLAY_MSG jmp to display loop
INT13OK: INT 13 ERROR
0000:0678 BEC206 MOV SI,06C2 "missing op sys"
0000:067B BFFE7D MOV DI,7DFE point to signature
0000:067E 813D55AA CMP WORD PTR [DI],AA55 is signature correct?
0000:0682 75C7 JNZ DISPLAY_MSG no
0000:0684 8BF5 MOV SI,BP set SI
0000:0686 EA007C0000 JMP 0000:7C00 JUMP TO THE BOOT SECTOR
WITH SI POINTING TO
PART TABLE ENTRY
Messages here.
0000:0680 ........ ........ ......49 6e76616c * Inval*
0000:0690 69642070 61727469 74696f6e 20746162 *id partition tab*
0000:06a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000:06b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000:06c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000:06d0 696e6720 73797374 656d00.. ........ *ing system. *
Data not used.
0000:06d0 ........ ........ ......00 00000000 * .....*
0000:06e0 00000000 00000000 00000000 00000000 *................*
0000:06f0 00000000 00000000 00000000 00000000 *................*
0000:0700 00000000 00000000 00000000 00000000 *................*
0000:0710 00000000 00000000 00000000 00000000 *................*
0000:0720 00000000 00000000 00000000 00000000 *................*
0000:0730 00000000 00000000 00000000 00000000 *................*
0000:0740 00000000 00000000 00000000 00000000 *................*
0000:0750 00000000 00000000 00000000 00000000 *................*
0000:0760 00000000 00000000 00000000 00000000 *................*
0000:0770 00000000 00000000 00000000 00000000 *................*
0000:0780 00000000 00000000 00000000 00000000 *................*
0000:0790 00000000 00000000 00000000 00000000 *................*
0000:07a0 00000000 00000000 00000000 00000000 *................*
0000:07b0 00000000 00000000 00000000 0000.... *............ *
The partition table starts at 0000:07be. Each partition table
entry is 16 bytes. This table defines a single primary partition
which is also an active (bootable) partition.
0000:07b0 ........ ........ ........ ....8001 * ....*
0000:07c0 0100060d fef83e00 00000678 0d000000 *......>....x....*
0000:07d0 00000000 00000000 00000000 00000000 *................*
0000:07e0 00000000 00000000 00000000 00000000 *................*
0000:07f0 00000000 00000000 00000000 0000.... *............ *
The last two bytes contain a 55AAH signature.
0000:07f0 ........ ........ ........ ....55aa *..............U.*
/end/
NOTICE (18 Dec 99): See the file README.TXT!
How It Works -- OS2 Boot Sector
Version 1a
by Hale Landis <hlandis@ibm.net>
OS2 BOOT SECTOR
Note: I'll leave it to someone else to provide you with a
disassembly of an OS/2 HPFS boot sector, or a Linux boot sector,
or a WinNT boot sector, etc.
This article is a disassembly of a floppy or hard disk boot
sector for OS/2. Apparently OS/2 uses the same boot sector for
both environments. Basically a bootable FAT hard disk partition
looks like a big floppy during the early stages of the system's
boot processing. This sector is at cylinder 0, head 0, sector 1
of a floppy or it is the first sector within a FAT hard disk
partition. OS/2 floppy disk and hard disk boot sectors are
created by the OS/2 FORMAT program.
At the completion of your system's Power On Self Test (POST), INT
19 is called. Usually INT 19 tries to read a boot sector from
the first floppy drive. If a boot sector is found on the floppy
disk, the that boot sector is read into memory at location
0000:7C00 and INT 19 jumps to memory location 0000:7C00.
However, if no boot sector is found on the first floppy drive,
INT 19 tries to read the MBR from the first hard drive. If an
MBR is found it is read into memory at location 0000:7c00 and INT
19 jumps to memory location 0000:7c00. The small program in the
MBR will attempt to locate an active (bootable) partition in its
partition table. If such a partition is found, the boot sector
of that partition is read into memory at location 0000:7C00 and
the MBR program jumps to memory location 0000:7C00. Each
operating system has its own boot sector format. The small
program in the boot sector must locate the first part of the
operating system's kernel loader program (or perhaps the kernel
itself or perhaps a "boot manager program") and read that into
memory.
INT 19 is also called when the CTRL-ALT-DEL keys are used. On
most systems, CTRL-ALT-DEL causes an short version of the POST to
be executed before INT 19 is called.
=====
Where stuff is:
The BIOS Parameter Block (BPB) starts at offset 0.
The boot sector program starts at offset 46.
The messages issued by this program start at offset 198.
The OS/2 boot loader file name starts at offset 1d5.
The boot sector signature is at offset 1fe.
Here is a summary of what this thing does:
1) If booting from a hard disk partition, skip to step 6.
2) Copy Diskette Parameter Table which is pointed to by INT 1E
to the top of memory.
3) Alter the copy of the Diskette Parameter Table.
4) Alter INT 1E to point to altered Diskette Parameter Table at
the top of memory.
5) Do INT 13 AH=00, disk reset call so that the altered
Diskette Parameter Table is used.
6) Compute sector address of the root directory.
7) Read the entire root directory into memory starting at
location 1000:0000.
8) Search the root directory entires for the file OS2BOOT.
9) Read the OS2BOOT file into memory at 0800:0000.
10) Do a far return to enter the OS2BOOT program at 0800:0000.
NOTES:
This program uses the CHS based INT 13H AH=02 to read the FAT
root directory and to read the OS2BOOT file. If the drive is
>528MB, this CHS must be a translated CHS (or L-CHS, see my
BIOS TYPES document). Except for internal computations no
addresses in LBA form are used, another reason why LBA doesn't
solve the >528MB problem.
=====
Here is the entire sector in hex and ascii.
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 eb449049 424d2032 302e3000 02100100 *.D.IBM 20.0.....*
000010 02000200 00f8d800 3e000e00 3e000000 *........>...>...*
000020 06780d00 80002900 1c0c234e 4f204e41 *.x....)...#NO NA*
000030 4d452020 20204641 54202020 20200000 *ME FAT ..*
000040 00100000 0000fa33 db8ed3bc ff7bfbba *.......3.....{..*
000050 c0078eda 803e2400 00753d1e b840008e *.....>$..u=..@..*
000060 c026ff0e 1300cd12 c1e0068e c033ff33 *.&...........3.3*
000070 c08ed8c5 367800fc b90b00f3 a41fa118 *....6x..........*
000080 0026a204 001e33c0 8ed8a378 008c067a *.&....3....x...z*
000090 001f8a16 2400cd13 a0100098 f7261600 *....$........&..*
0000a0 03060e00 5091b820 00f72611 008b1e0b *....P.. ..&.....*
0000b0 0003c348 f7f35003 c1a33e00 b800108e *...H..P...>.....*
0000c0 c033ff59 890e4400 58a34200 33d2e873 *.3.Y..D.X.B.3..s*
0000d0 0033db8b 0e11008b fb51b90b 00bed501 *.3.......Q......*
0000e0 f3a65974 0583c320 e2ede335 268b471c *..Yt... ...5&.G.*
0000f0 268b571e f7360b00 fec08ac8 268b571a *&.W..6......&.W.*
000100 4a4aa00d 0032e4f7 e203063e 0083d200 *JJ...2.....>....*
000110 bb00088e c333ff06 57e82800 8d360b00 *.....3..W.(..6..*
000120 cbbe9801 eb03bead 01e80900 bec201e8 *................*
000130 0300fbeb feac0ac0 7409b40e bb0700cd *........t.......*
000140 10ebf2c3 50525103 061c0013 161e00f7 *....PRQ.........*
000150 361800fe c28ada33 d2f7361a 008afa8b *6......3..6.....*
000160 d0a11800 2ac34050 b402b106 d2e60af3 *....*.@P........*
000170 8bca86e9 8a162400 8af78bdf cd1372a6 *......$.......r.*
000180 5b598bc3 f7260b00 03f85a58 03c383d2 *[Y...&....ZX....*
000190 002acb7f afc31200 4f532f32 20212120 *.*......OS/2 !! *
0001a0 53595330 31343735 0d0a0012 004f532f *SYS01475.....OS/*
0001b0 32202121 20535953 30323032 350d0a00 *2 !! SYS02025...*
0001c0 12004f53 2f322021 21205359 53303230 *..OS/2 !! SYS020*
0001d0 32370d0a 004f5332 424f4f54 20202020 *27...OS2BOOT *
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
=====
The first 62 bytes of a boot sector are known as the BIOS
Parameter Block (BPB). Here is the layout of the BPB fields
and the values they are assigned in this boot sector:
db JMP instruction at 7c00 size 2 = eb44
db NOP instruction 7c02 1 90
db OEMname 7c03 8 'IBM 20.0'
dw bytesPerSector 7c0b 2 0200
db sectPerCluster 7c0d 1 01
dw reservedSectors 7c0e 2 0001
db numFAT 7c10 1 02
dw numRootDirEntries 7c11 2 0200
dw numSectors 7c13 2 0000 (use numSectorsHuge)
db mediaType 7c15 1 f8
dw numFATsectors 7c16 2 00d8
dw sectorsPerTrack 7c18 2 003e
dw numHeads 7c1a 2 000e
dd numHiddenSectors 7c1c 4 00000000
dd numSectorsHuge 7c20 4 000d7806
db driveNum 7c24 1 80
db reserved 7c25 1 00
db signature 7c26 1 29
dd volumeID 7c27 4 001c0c23
db volumeLabel 7c2b 11 'NO NAME '
db fileSysType 7c36 8 'FAT '
=====
Here is the boot sector...
The first 3 bytes of the BPB are JMP and NOP instructions.
0000:7C00 EB44 JMP START
0000:7C02 90 NOP
Here is the rest of the BPB.
0000:7C00 eb449049 424d2032 302e3000 02100100 *.D.IBM 20.0.....*
0000:7C10 02000200 00f8d800 3e000e00 3e000000 *........>...>...*
0000:7C20 06780d00 80002900 1c0c234e 4f204e41 *.x....)...#NO NA*
0000:7C30 4d452020 20204641 54202020 20200000 *ME FAT ..*
Additional data areas.
0000:7C30 ........ ........ ........ ....0000 * ..*
0000:7C40 00100000 0000.... ........ ........ *...... *
Note:
0000:7c3e (DS:003e) = number of sectors in the FATs and root dir.
0000:7c42 (DS:0042) = number of sectors in the FAT.
0000:7c44 (DS:0044) = number of sectors in the root dir.
START: START OF BOOT SECTOR PROGRAM
0000:7C46 FA CLI interrupts off
0000:7C47 33DB XOR BX,BX zero BX
0000:7C49 8ED3 MOV SS,BX SS now zero
0000:7C4B BCFF7B MOV SP,7BFF SP now 7bff
0000:7C4E FB STI interrupts on
0000:7C4F BAC007 MOV DX,07C0 set DX to
0000:7C52 8EDA MOV DS,DX 07c0
Are we booting from a floppy or a
hard disk partition?
0000:7C54 803E240000 CMP BYTE PTR [0024],00 is driveNum in BPB 00?
0000:7C59 753D JNZ NOT_FLOPPY jmp if not zero
We are booting from a floppy. The
Diskette Parameter Table must be
copied and altered...
Diskette Parameter Table is pointed to by INT 1E. This
program moves this table to high memory, alters the table, and
changes INT 1E to point to the altered table.
This table contains the following data:
????:0000 = Step rate and head unload time.
????:0001 = Head load time and DMA mode flag.
????:0002 = Delay for motor turn off.
????:0003 = Bytes per sector.
????:0004 = Sectors per track.
????:0005 = Intersector gap length.
????:0006 = Data length.
????:0007 = Intersector gap length during format.
????:0008 = Format byte value.
????:0009 = Head settling time.
????:000a = Delay until motor at normal speed.
Compute a valid high memory address.
0000:7C5B 1E PUSH DS save DS
0000:7C5C B84000 MOV AX,0040 set ES
0000:7C5F 8EC0 MOV ES,AX to 0040 (BIOS data area)
0000:7C61 26 ES: reduce system memory
0000:7C62 FF0E1300 DEC WORD PTR [0013] size by 1024
0000:7C66 CD12 INT 12 get system memory size
0000:7C68 C1E06 SHL AX,06 shift AX (mult by 64)
0000:7C6B 8EC0 MOV ES,AX move to ES
0000:7C6D 33FF XOR DI,DI zero DI
Move the diskette param table to high memory.
0000:7C6F 33C0 XOR AX,AX zero AX
0000:7C71 8ED8 MOV DS,AX DS now zero
0000:7C73 C5367800 LDS SI,[0078] DS:SI = INT 1E vector
0000:7C77 FC CLD clear direction
0000:7C78 B90B00 MOV CX,000B count is 11
0000:7C7B F3 REPZ copy diskette param table
0000:7C7C A4 MOVSB to top of memory
Alter the number of sectors per track
in the diskette param table in high memory.
0000:7C7D 1F POP DS restore DS
0000:7C7E A11800 MOV AX,[0018] get sectorsPerTrack from BPB
0000:7C81 26 ES: alter sectors per track
0000:7C82 A20400 MOV [0004],AL in diskette param table
Change INT 1E to point to altered diskette
param table and do a INT 13 disk reset call.
0000:7C85 1E PUSH DS save DS
0000:7C86 33C0 XOR AX,AX AX now zero
0000:7C88 8ED8 MOV DS,AX DS no zero
0000:7C8A A37800 MOV [0078],AX alter INT 1E vector
0000:7C8D 8C067A00 MOV [007A],ES to point to altered
diskette param table
0000:7C91 1F POP DS restore DS
0000:7C92 8A162400 MOV DL,[0024] driveNum from BPB
0000:7C96 CD13 INT 13 diskette reset
NOT_FLOPPY:
Compute the location and the size of
the root directory. Read the entire
root directory into memory.
0000:7C98 A01000 MOV AL,[0010] get numFAT
0000:7C9B 98 CBW make into a word
0000:7C9C F7261600 MUL WORD PTR [0016] mult by numFatSectors
0000:7CA0 03060E00 ADD AX,[000E] add reservedSectors
0000:7CA4 50 PUSH AX save
0000:7CA5 91 XCHG CX,AX move to CX
0000:7CA6 B82000 MOV AX,0020 dir entry size
0000:7CA9 F7261100 MUL WORD PTR [0011] mult by numRootDirEntries
0000:7CAD 8B1E0B00 MOV BX,[000B] get bytesPerSector
0000:7CB1 03C3 ADD AX,BX add
0000:7CB3 48 DEC AX subtract 1
0000:7CB4 F7F3 DIV BX div by bytesPerSector
0000:7CB6 50 PUSH AX save number of dir sectors
0000:7CB7 03C1 ADD AX,CX add number of fat sectors
0000:7CB9 A33E00 MOV [003E],AX save
0000:7CBC B80010 MOV AX,1000 AX is now 1000
0000:7CBF 8EC0 MOV ES,AX ES is now 1000
0000:7CC1 33FF XOR DI,DI DI is now zero
0000:7CC3 59 POP CX get number dir sectors
0000:7CC4 890E4400 MOV [0044],CX save
0000:7CC8 58 POP AX get number fat sectors
0000:7CC9 A34200 MOV [0042],AX save
0000:7CCC 33D2 XOR DX,DX DX now zero
0000:7CCE E87300 CALL READ_SECTOR read 1st sect of root dir
0000:7CD1 33DB XOR BX,BX BX is now zero
0000:7CD3 8B0E1100 MOV CX,[0011] number of root dir entries
DIR_SEARCH: SEARCH FOR OS2BOOT.
Search the root directory for the file
name OS2BOOT.
0000:7CD7 8BFB MOV DI,BX DI is dir entry addr
0000:7CD9 51 PUSH CX save CX
0000:7CDA B90B00 MOV CX,000B count is 11
0000:7CDD BED501 MOV SI,01D5 addr of "OS2BOOT"
0000:7CE0 F3 REPZ is 1st dir entry
0000:7CE1 A6 CMPSB for "OS2BOOT"?
0000:7CE2 59 POP CX restore CX
0000:7CE3 7405 JZ FOUND_OS2BOOT jmp if OS2BOOT
0000:7CE5 83C320 ADD BX,+20 incr to next dir entry
0000:7CE8 E2ED LOOP DIR_SEARCH try again
FOUND_OS2BOOT: FOUND OS2BOOT.
OS2BOOT was found. Get the starting
cluster number and convert to a sector
address. Read OS2BOOT into memory and
finally do a far return to enter
the OS2BOOT program.
0000:7CEA E335 JCXZ FAILED1 JMP if CX zero.
0000:7CEC 26 ES: get the szie of
0000:7CED 8B471C MOV AX,[BX+1C] the OS2BOOT file
0000:7CF0 26 ES: from the OS2BOOT
0000:7CF1 8B571E MOV DX,[BX+1E] directory entry
0000:7CF4 F7360B00 DIV WORD PTR [000B] div by bytesPerSect
0000:7CF8 FEC0 INC AL add 1
0000:7CFA 8AC8 MOV CL,AL num sectors OS2BOOT
0000:7CFC 26 ES: get the starting
0000:7CFD 8B571A MOV DX,[BX+1A] cluster number
0000:7D00 4A DEC DX subtract 1
0000:7D01 4A DEC DX subtract 1
0000:7D02 A00D00 MOV AL,[000D] sectorsPerCluster
0000:7D05 32E4 XOR AH,AH mutiply
0000:7D07 F7E2 MUL DX to get LBA
0000:7D09 03063E00 ADD AX,[003E] add number of FAT sectors
0000:7D0D 83D200 ADC DX,+00 to LBA
0000:7D10 BB0008 MOV BX,0800 set ES
0000:7D13 8EC3 MOV ES,BX to 0800
0000:7D15 33FF XOR DI,DI set ES:DI to entry point
0000:7D17 06 PUSH ES address of
0000:7D18 57 PUSH DI OS2BOOT
0000:7D19 E82800 CALL READ_SECTOR read OS2BOOT into memory
0000:7D1C 8D360B00 LEA SI,[000B] set DS:SI
0000:7D20 CB RETF "far return" to OS2BOOT
FAILED1: OS2BOOT WAS NOT FOUND.
0000:7D21 BE9801 MOV SI,0198 "SYS01475" message
0000:7D24 EB03 JMP FAILED3
FAILED2: ERROR FROM INT 13.
0000:7D26 BEAD01 MOV SI,01AD "SYS02025" message
FAILED3: OUTPUT ERROR MESSAGES.
0000:7D29 E80900 CALL MSG_LOOP display message
0000:7D2C BEC201 MOV SI,01C2 "SYS02027" message
0000:7D2F E80300 CALL MSG_LOOP display message
0000:7D32 FB STI interrupts on
HANG: HANG THE SYSTEM!
0000:7D33 EBFE JMP HANG sit and stay!
MSG_LOOP: DISPLAY AN ERROR MESSAGE.
Routine to display the message
text pointed to by SI.
0000:7D35 AC LODSB get next char of message
0000:7D36 0AC0 OR AL,AL end of message?
0000:7D38 7409 JZ RETURN jmp if yes
0000:7D3A B40E MOV AH,0E write 1 char
0000:7D3C BB0700 MOV BX,0007 video attributes
0000:7D3F CD10 INT 10 INT 10 to write 1 char
0000:7D41 EBF2 JMP MSG_LOOP do again
RETURN:
0000:7D43 C3 RET return
READ_SECTOR: ROUTINE TO READ SECTORS.
Read sectors into memory. Read multiple
sectors but don't read across a track
boundary.
The caller supplies the following:
DX:AX = sector address to read (as LBA)
CX = number of sectors to read
ES:DI = memory address to read into
0000:7D44 50 PUSH AX save lower part of LBA
0000:7D45 52 PUSH DX save upper part of LBA
0000:7D46 51 PUSH CX save number of sect to read
0000:7D47 03061C00 ADD AX,[001C] add numHiddenSectors
0000:7D4B 13161E00 ADC DX,[001E] to LBA
0000:7D4F F7361800 DIV WORD PTR [0018] div by sectorsPerTrack
0000:7D53 FEC2 INC DL add 1 to sector number
0000:7D55 8ADA MOV BL,DL save sector number
0000:7D57 33D2 XOR DX,DX zero upper part of LBA
0000:7D59 F7361A00 DIV WORD PTR [001A] div by numHeads
0000:7D5D 8AFA MOV BH,DL save head number
0000:7D5F 8BD0 MOV DX,AX save cylinder number
0000:7D61 A11800 MOV AX,[0018] sectorsPerTrack
0000:7D64 2AC3 SUB AL,BL sub sector number
0000:7D66 40 INC AX add 1
0000:7D67 50 PUSH AX save number of sector to read
0000:7D68 B402 MOV AH,02 INT 13 read sectors
0000:7D6A B106 MOV CL,06 shift count
0000:7D6C D2E6 SHL DH,CL shift high cyl left
0000:7D6E 0AF3 OR DH,BL or in sector number
0000:7D70 8BCA MOV CX,DX move cyl/sect to CX
0000:7D72 86E9 XCHG CH,CL swap cyl/sect
0000:7D74 8A162400 MOV DL,[0024] driveNum
0000:7D78 8AF7 MOV DH,BH head number
0000:7D7A 8BDF MOV BX,DI memory addr to read into
0000:7D7C CD13 INT 13 INT 13 read sectors call
0000:7D7E 72A6 JB FAILED2 jmp if any error
0000:7D80 5B POP BX get number of sectors read
0000:7D81 59 POP CX restore CX
0000:7D82 8BC3 MOV AX,BX number of sector to AX
0000:7D84 F7260B00 MUL WORD PTR [000B] multiply by sector size
0000:7D88 03F8 ADD DI,AX add to memory address
0000:7D8A 5A POP DX restore upper part of LBA
0000:7D8B 58 POP AX resotre lower part of LBA
0000:7D8C 03C3 ADD AX,BX add number of sector just
0000:7D8E 83D200 ADC DX,+00 read to LBA
0000:7D91 2ACB SUB CL,BL decr requested num of sect
0000:7D93 7FAF JG READ_SECTOR jmp if not zero
0000:7D95 C3 RET return
Data not used.
0000:7D90 ........ ....1200 ........ ........ * .. *
Messages here.
0000:7D90 ........ ........ 4f532f32 20212120 * OS/2 !! *
0000:7Da0 53595330 31343735 0d0a0012 004f532f *SYS01475.....OS/*
0000:7Db0 32202121 20535953 30323032 350d0a00 *2 !! SYS02025...*
0000:7Dc0 12004f53 2f322021 21205359 53303230 *..OS/2 !! SYS020*
0000:7Dd0 32370d0a 00...... ........ ........ *27... *
OS/2 loader file name.
0000:7Dd0 ........ ..4f5332 424f4f54 20202020 * OS2BOOT *
Data not used.
0000:7De0 00000000 00000000 00000000 00000000 *................*
0000:7Df0 00000000 00000000 00000000 0000.... *.............. *
The last two bytes contain a 55AAH signature.
0000:7Df0 ........ ........ ........ ....55aa * U.*
/end/
NOTICE (18 Dec 99): See the file README.TXT!
How it Works -- Partition Tables
Part 1 of 2
Version 1i -- 24 Jun 99
by Hale Landis <hlandis@ibm.net>
NOTE: A "!" at the left margin of the first line of a paragraph
indicates that some change was made in that paragraph.
PARTITION SECTOR/RECORD/TABLE BASICS
FDISK creates all partition records (sectors). The primary
purpose of a partition record is to hold a partition table. The
rules for how FDISK works are unwritten but so far most FDISK
programs (DOS, OS/2, WinNT, etc) seem to follow the same basic
idea.
First, all partition table records (sectors) have the same
format. This includes the partition table record at cylinder 0,
head 0, sector 1 -- what is known as the Master Boot Record
(MBR). The last 66 bytes of a partition table record contain a
partition table and a 2 byte signature. The first 446 bytes of
these sectors usually contain a program but only the program in
the MBR is ever executed (so extended partition table records
could contain something other than a program in the first 466
bytes). See "How It Works -- The Master Boot Record".
Second, extended partitions are "nested" inside one another and
extended partition table records form a "linked list". I will
attempt to show this in a diagram below.
PARTITION TABLE ENTRY FORMAT
Each partition table entry is 16 bytes and contains things like
the start and end location of a partition in CHS, the start in
LBA, the size in sectors, the partition "type" and the "active"
flag. Warning: older versions of FDISK may compute incorrect
LBA or size values. And note: When your computer boots itself,
only the CHS fields of the partition table entries are used
(another reason LBA doesn't solve the >528MB problem). The CHS
fields in the partition tables are in L-CHS format -- see "How It
Works -- CHS Translation".
! WARNING: Apparently a "bug" in some recent version of FDISK
has now become the "standard". The Starting LBA field used to be
the "absolute" LBA and was equivalent to the Starting CHS.
However, newer versions of FDISK now set the Starting LBA field
of a secondary partition entry to the offset from the start of
the extended partition. This is really stupid and it is just
another example of the partition table and FDISK chaos. If the
starting LBA field for a secondary partition is some really small
value then it is probably an offset. In this case add this
offset to the Starting LBA of the extended partition to
get the absolute Starting LBA of the secondary partition.
! ANOTHER WARNING: CHS addressing only works up to about 8GB.
The actual value is 7.9GB for 8.4GB depending on the version of
FDISK and the OS(s) you are running. This corresponds to a
translated CHS with 1024 cylinders, 240 heads and 63 sectors
(7.9GB) or 1024 cylinders, 256 heads and 63 sectors (8.4GB).
With most new disk drives being over 8GB, CHS addressing really
is obsolete but the folks implementing FDISK have been slow to
respond to this. The result is when a partition is beyond
(either starts or spans) the 8GB boundary, there is no "standard"
for what value should be placed into the Starting CHS and Ending
CHS fields. A value of all 1 bits, equal to cylinder 1023 (or
3FFH) head 15 (or FH) sector 63 (or 3FH), is common but by no
means a "standard". If you see a Starting CHS value that makes
no sense, then probably you should ignore the CHS information and
use only the Starting LBA.
! ALSO NOTE: The Ending CHS and Size in Sectors fields are
really mostly useless and are frequently set to incorrect values
by various FDISK programs. These fields really are not that
important because in most cases you can look at the first few
sectors of a partition and see what kind of file system is there
and how big it is. This information is always more correct than
the Ending CHS and Size in Sectors fields.
There is no central clearing house to assign the codes used in
the one byte "type" field, however, there is at least one person
at both Microsoft and IBM that attempt to keep track of the type
codes. Type codes are used to define most every type of file
system that anyone has ever implemented on the x86 PC: 12-bit
FAT, 16-bit FAT, HPFS, NTFS, etc. Plus, an extended partition
also has a unique type code. The complete list of known
partition type codes is contained in part 2 of this document.
The 16 bytes of a partition table entry are used as follows:
+--- Bit 7 is the active partition flag, bits 6-0 are zero
| (when not zero this byte is also the drive
| number of the drive to boot so the active
| partition is always found on drive 80H,
| the first hard disk).
|
| +--- Starting CHS in INT 13 call format.
| |
| | +--- Partition type byte.
| | |
| | | +--- Ending CHS in INT 13 call format.
| | | |
| | | | +-- Starting LBA.
| | | | |
| | | | | +-- Size in Sectors.
| | | | | |
v <--+---> v <--+--> v v
0 1 2 3 4 5 6 7 8 9 A B C D E F
DL DH CL CH TB DH CL CH LBA..... SIZE....
80 01 01 00 06 0e be 94 3e000000 0c610900 1st entry
00 00 81 95 05 0e fe 7d 4a610900 724e0300 2nd entry
00 00 00 00 00 00 00 00 00000000 00000000 3rd entry
00 00 00 00 00 00 00 00 00000000 00000000 4th entry
Bytes 0-3 are used by the small program in the Master Boot Record
to read the first sector of an active partition into memory. The
DH, DL, CH and CL above show which x86 register is loaded when
the MBR program calls INT 13H AH=02H to read the active
partition's boot sector. See "How It Works -- Master Boot
Record".
These entries define the following partitions:
1) The first partition, a primary partition DOS FAT, starts at
CHS 0H,1H,1H (LBA 3EH) and ends at CHS 294H,EH,3EH with a size
of 9610CH sectors.
2) The second partition, an extended partition, starts at CHS
295H,0H,1H (LBA 9614AH) and ends at CHS 37DH,EH,3EH with a
size of 34E72H sectors.
3) The third and fourth table entries are unused.
PARTITION TABLE RULES
Keep in mind that there are NO written rules and NO industry
standards on how FDISK should work but here are some basic rules
that seem to be followed by most versions of FDISK:
1) In the MBR there can be 0-4 "primary" partitions, OR, 0-3
primary partitions and 0-1 extended partition entry.
2) In an extended partition there can be 0-1 "secondary"
partition entries and 0-1 extended partition entries.
3) Only 1 primary partition in the MBR can be marked "active" at
any given time.
4) In most versions of FDISK, the first sector of a partition
will be aligned such that it is at head 0, sector 1 of a
cylinder. This means that there may be unused sectors on the
track(s) prior to the first sector of a partition and that
there may be unused sectors following a partition table
sector.
For example, most new versions of FDISK start the first
partition (primary or extended) at cylinder 0, head 1, sector
1. This leaves the sectors at cylinder 0, head 0, sectors
2...n as unused sectors. This same layout may be seen on the
first track of an extended partition. See example 2 below.
Also note that software drivers like Ontrack's Disk Manager
depend on these unused sectors because these drivers will
"hide" their code there (in cylinder 0, head 0, sectors
2...n). This is also a good place for boot sector virus
programs to hang out.
5) The partition table entries (slots) can be used in any order.
Some versions of FDISK fill the table from the bottom up and
some versions of FDISK fill the table from the top down.
Deleting a partition can leave an unused entry (slot) in the
middle of a table.
6) And then there is the "hack" that some newer OS's (OS/2 and
Linux) use in order to place a partition spanning or passed
cylinder 1024 on a system that does not have a CHS translating
BIOS. These systems create a partition table entry with the
partition's starting and ending CHS information set to all
FFH. The starting and ending LBA information is used to
describe the location of the partition. The LBA can be
converted back to a CHS -- most likely a CHS with more than
1024 cylinders. Since such a CHS can't be used by the system
BIOS, these partitions can not be booted or accessed until the
OS's kernel and hard disk device drivers are loaded. It is
not known if the systems using this "hack" follow the same
rules for the creation of these type of partitions.
There are NO written rules as to how an OS scans the partition
table entries so each OS can have a different method. For DOS,
this means that different versions could assign different drive
letters to the same FAT file system partitions.
PARTITION NESTING
What do I mean when I say the partitions are "nested" within each
other? Lets look at this example:
M = Master Boot Record (and any unused sectors
on the same track)
E = Extended partition record (and any unused sectors
on the same track)
pri = a primary partition (first sector is a "boot" sector)
sec = a secondary partition (first sector is a "boot" sector)
|<----------------the entire disk-------------->|
| |
|M<pri> |
| |
| E<sec><---rest of 1st ext part---------->|
| |
| E<sec><---rest of 2nd ext part---->|
The first extended partition is described in the MBR and it
occupies the entire disk following the primary partition. The
second extended partition is described in the first extended
partition record and it occupies the entire disk following the
first secondary partition.
PARTITION TABLE LINKING
What do I mean when I say the partition records (tables) form a
"linked" list? This means that the MBR has an entry that
describes (points to) the first extended partition, the first
extended partition table has an entry that describes (points to)
the second extended partition table, and so on. There is, in
theory, no limited to out long this linked list is. When you ask
FDISK to show the DOS "logical drives" it scans the linked list
looking for all of the DOS FAT type partitions that may exist.
Remember that in an extended partition table, only two entries of
the four can be used (rule 2 above).
And one more thing... Within a partition, the layout of the file
system data varies greatly. However, the first sector of a
partition is expected to be a "boot" sector. A DOS FAT file
system has: a boot sector, first FAT sectors, second FAT
sectors, root directory sectors and finally the file data area.
See "How It Works -- OS2 Boot Sector".
EXAMPLE 1
A disk containing four DOS FAT partitions (C, D, E and F):
|<---------------------the entire disk------------------->|
| |
|M<---C:---> |
| |
| E<---D:---><-rest of 1st ext part------------>|
| |
| E<---E:---><-rest of 2nd ext part->|
| |
| E<---------F:---------->|
EXAMPLE 2
So here is an example of a disk with two primary partitions, one
DOS FAT and one OS/2 HPFS, plus an extended partition with
another DOS FAT:
|<------------------the entire disk------------------>|
| |
|M<pri 1 - DOS FAT> |
| |
| <pri 2 - OS/2 HPFS> |
| |
| E<sec - DOS FAT>|
Or in more detail ('n' is the highest cylinder, head or sector
number number allowed in the indicated field of the CHS)...
+-------------------------------------+
CHS=0,0,1 | Master Boot Record containing |
| partition table search program and |
| a partition table |
| +---------------------------------+ |
| | DOS FAT partition description | | points to CHS=0,1,1
| +---------------------------------+ | points to CHS=a
| | OS/2 HPFS partition description | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | extended partition entry | | points to CHS=b
| +---------------------------------+ |
+-------------------------------------+
CHS=0,0,2 | the rest of "track 0" -- this is | :
to | where the software drivers such as | : normally
CHS=0,0,n | Ontrack's Disk Manager or Micro | : unused
| House's EZ Drive are located. | :
+-------------------------------------+
CHS=0,1,1 | Boot sector for the DOS FAT | :
| partition | : a DOS FAT
+-------------------------------------+ : file
CHS=0,1,2 | rest of the DOS FAT partition | : system
to | (FAT table, root directory and | :
CHS=x-1,n,n | user data area) | :
+-------------------------------------+
CHS=x,0,1 | Boot sector for the OS/2 HPFS | :
| file system partition | : an OS/2
+-------------------------------------+ : HPFS file
CHS=x,0,2 | rest of the OS/2 HPFS file system | : system
to | partition | :
CHS=y-1,n,n | | :
+-------------------------------------+
CHS=y,0,1 | Partition record for the extended |
| partition containing a partition |
| record program (never executed) and |
| a partition table |
| +---------------------------------+ |
| | DOS FAT partition description | | points to CHS=b+1
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
+-------------------------------------+
CHS=y,0,2 | the rest of the first track of the | : normally
to | extended partition | : unused
CHS=y,0,n | | :
+-------------------------------------+
CHS=y,1,1 | Boot sector for the DOS FAT | :
| partition | : a DOS FAT
+-------------------------------------+ : file
CHS=y,1,2 | rest of the DOS FAT partition | : system
to | (FAT table, root directory and | :
CHS=n,n,n | user data area) | :
+-------------------------------------+
EXAMPLE 3
Here is a partition record from an extended partition (the first
sector of an extended partition). Note that it contains no
program code. It contains only the partition table and the
signature data.
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 00000000 00000000 00000000 00000000 *................*
000010 TO 0001af SAME AS ABOVE
0001b0 00000000 00000000 00000000 00000001 *................*
0001c0 8195060e fe7d3e00 0000344e 03000000 *.....}>...4N....*
0001d0 00000000 00000000 00000000 00000000 *................*
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
NOTES
Thanks to yue@heron.Stanford.EDU (Kenneth C. Yue) for pointing
out that in V0 of this document I did not properly describe the
unused sectors normally found around the partition table sectors.
Thanks to Marcus.Better@abc.se (Marcus Better) for pointing out
that in V1a-c of this document I did not properly describe the
x86 registers that the partition table entry data is loaded into
when INT 13 is called.
/end part 1 of 2/
NOTICE (18 Dec 99): See the file README.TXT!
How It Works -- CHS Translation
Plus BIOS Types, LBA and Other Good Stuff
Part 2 of 2
Version 4b -- 24 Jun 99
by Hale Landis <hlandis@ibm.net>
NOTE: A "!" at the left margin of the first line of a paragraph
indicates that some change was made in that paragraph.
/continued from part 1 of 2/
The BIOS Types
--------------
I assume the following:
a) All BIOS INT 13H support has been installed by the time the OS
starts its boot processing. I'm don't plan to cover what
could happen to INT 13H once the OS starts loading its own
device drivers.
b) Drives supported by INT 13H are numbered sequentially starting
with drive number 80H (80H-FFH are hard drives, 00-7FH are
floppy drives).
And remember, any time a P-CHS exists it may or may not account
for the CE Cylinder properly.
I have identified the following types of BIOS INT 13H support as
seen by an OS during its boot time hardware configuration
determination:
BIOS Type 1
Origin: Original IBM PC/XT.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. There is no CHS translation. INT 13H AH=08H
returns the P-CHS. The FDPT should contain the same P-CHS.
Description: Supports up to 528MB from a table of drive
descriptions in BIOS ROM. No support for >1024 cylinders or
drives >528MB or LBA.
Support issues: For >1024 cylinders or >528MB support, either
an option ROM with an INT 13H replacement (see BIOS types 4-7)
-or- a software driver (see BIOS type 8) must be added to the
system.
BIOS Type 2
Origin: Unknown, but first appeared on systems having BIOS
drive type table entries defining >1024 cylinders. Rumored to
have originated at the request of Novell or SCO.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H should return a L-CHS with the
cylinder value limited to 1024. Beware, many BIOS perform
a logical AND on the cylinder value. A correct BIOS will
limit the cylinder value as follows:
cylinder = cylinder > 1024 ? 1024 : cylinder;
An incorrect BIOS will limit the cylinder value as follows
(this implementation turns a 540MB drive into a 12MB drive!):
cylinder = cylinder & 0x03ff;
The FDPT will return a P-CHS that has the full cylinder
value.
Description: For BIOS drive numbers 80H and 81H, this BIOS
type supports >1024 cylinders or >528MB without using a
translated CHS in the FDPT. INT 13H AH=08H truncates
cylinders to 1024 (beware of buggy implementations). The FDPT
can show >1024 cylinders thereby allowing an OS to support
drives >528MB. May convert the L-CHS or P-CHS directly to an
LBA if the ATA device supports LBA.
Support issues: Actual support of >1024 cylinders is OS
specific -- some OS's may be able to place OS specific
partitions spanning or beyond cylinder 1024. Usually all OS
boot code must be within first 1024 cylinders. The FDISK
program of an OS that supports such partitions uses an OS
specific partition table entry format to identify these
paritions. There does not appear to be a standard (de facto
or otherwise) for this unusual partition table entry.
Apparently one method is to place -1 into the CHS fields and
use the LBA fields to describe the location of the partition.
This DOES NOT require the drive to support LBA addressing.
Using an LBA in the partition table entry is just a trick to
get around the CHS limits in the partition table entry. It is
unclear if such a partition table entry will be ignored by an
OS that does not understand what it is. For an OS that does
not support such partitions, either an option ROM with an INT
13H replacement (see BIOS types 4-7) -or- a software driver
(see BIOS type 8) must be added to the system.
Note: OS/2 can place HPFS partitions and Linux can place
Linux partitions beyond or spanning cylinder 1024. (Anyone
know of other systems that can do the same?)
BIOS Type 3
Origin: Unknown, but first appeared on systems having BIOS
drive type table entires defining >1024 cylinders. Rumored to
have originated at the request of Novell or SCO.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H can return an L-CHS with more
than 1024 cylinders.
Description: This BIOS is like type 2 above but it allows up
to 4096 cylinders (12 cylinder bits). It does this in the INT
13H AH=0xH calls by placing two most significant cylinder bits
(bits 11 and 10) into the upper two bits of the head number
(bits 7 and 6).
Support issues: Identification of such a BIOS is difficult.
As long as the drive(s) supported by this type of BIOS have
<1024 cylinders this BIOS looks like a type 2 BIOS because INT
13H AH=08H should return zero data in bits 7 and 6 of the head
information. If INT 13H AH=08H returns non zero data in bits
7 and 6 of the head information, perhaps it can be assumed
that this is a type 3 BIOS. For more normal support of >1024
cylinders or >528MB, either an option ROM with an INT 13H
replacement (see BIOS types 4-7) -or- a software driver (see
BIOS type 8) must be added to the system.
Note: Apparently this BIOS type is no longer produced by any
BIOS vendor.
BIOS Type 4
Origin: Compaq. Probably first appeared in systems with ESDI
drives having >1024 cylinders.
BIOS call support: INT 13H AH=0xH and EDPT for BIOS drives
80H and 81H. If the drive has <1024 cylinders, INT 13H AH=08H
returns the P-CHS and a FDPT is built. If the drive has >1024
cylinders, INT 13H AH=08H returns an L-CHS and an EDPT is
built.
Description: Looks like a type 2 BIOS when an FDPT is built.
Uses CHS translation when an EDPT is used. May convert the
L-CHS directly to an LBA if the ATA device supports LBA.
Support issues: This BIOS type may support up to four drives
with a EDPT (or FDPT) for BIOS drive numbers 82H and 83H
located in memory following the EDPT (or FDPT) for drive 80H.
Different CHS translation algorithms may be used by the BIOS
and an OS.
BIOS Type 5
Origin: The IBM/Microsoft BIOS Extensions document. For many
years this document was marked "confidential" so it did not
get wide spread distribution.
BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for BIOS
drives 80H and 81H. INT 13H AH=08H returns an L-CHS. INT 13H
AH=41H and AH=48H should be used to get P-CHS configuration.
The FDPT/EDPT should not be used. In some implementations the
FDPT/EDPT may not exist.
Description: A BIOS that supports very large drives (>1024
cylinders, >528MB, actually >8GB), and supports the INT 13H
AH=4xH read/write functions. The AH=4xH calls use LBA
addressing and support drives with up to 2^64 sectors. These
calls do NOT require that a drive support LBA at the drive
interface. INT 13H AH=48H describes the L-CHS used at the INT
13 interface and the P-CHS or LBA used at the drive interface.
This BIOS supports the INT 13 AH=0xH calls the same as a BIOS
type 4.
Support issues: While the INT 13H AH=4xH calls are well
defined, they are not implemented in many systems shipping
today. Currently undefined is how such a BIOS should respond
to INT 13H AH=08H calls for a drive that is >8GB. Different
CHS translation algorithms may be used by the BIOS and an OS.
Note: Support of LBA at the drive interface may be automatic
or may be under user control via a BIOS setup option. Use of
LBA at the drive interface does not change the operation of
the INT 13 interface.
BIOS Type 6
Origin: The Phoenix Enhanced Disk Drive Specification.
BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for BIOS
drives 80H and 81H. INT 13H AH=08H returns an L-CHS. INT 13H
AH=41H and AH=48H should be used to get P-CHS configuration.
INT 13H AH=48H returns the address of the Phoenix defined
"FDPT Extension" table.
Description: A BIOS that supports very large drives (>1024
cylinders, >528MB, actually >8GB), and supports the INT 13H
AH=4xH read/write functions. The AH=4xH calls use LBA
addressing and support drives with up to 2^64 sectors. These
calls do NOT require that a drive support LBA at the drive
interface. INT 13H AH=48H describes the L-CHS used at the INT
13 interface and the P-CHS or LBA used at the drive interface.
This BIOS supports the INT 13 AH=0xH calls the same as a BIOS
type 4. The INT 13H AH=48H call returns additional information
such as host adapter addresses, DMA support, LBA support, etc,
in the Phoenix defined "FDPT Extension" table.
Phoenix says this this BIOS need not support the INT 13H
AH=4xH read/write calls but this BIOS is really an
extension/enhancement of the original IBM/MS BIOS so most
implementations will probably support the full set of INT 13H
AH=4xH calls.
Support issues: Currently undefined is how such a BIOS should
respond to INT 13H AH=08H calls for a drive that is >8GB.
Different CHS translation algorithms may be used by the BIOS
and an OS.
Note: Support of LBA at the drive interface may be automatic
or may be under user control via a BIOS setup option. Use of
LBA at the drive interface does not change the operation of
the INT 13 interface.
BIOS Type 7
Origin: Described in the Western Digital Enhanced IDE
Implementation Guide.
BIOS call support: INT 13H AH=0xH and FDPT or EDPT for BIOS
drives 80H and 81H. An EDPT with a L-CHS of 16 heads and 63
sectors is built when "LBA mode" is enabled. An FDPT is built
when "LBA mode" is disabled.
Description: Supports >1024 cylinders or >528MB using a EDPT
with a translated CHS *** BUT ONLY IF *** the user requests
"LBA mode" in the BIOS setup *** AND *** the drive supports
LBA. As long as "LBA mode" is enabled, CHS translation is
enabled using a L-CHS with <=1024 cylinders, 16, 32, 64, ...,
heads and 63 sectors. Disk read/write commands are issued in
LBA mode at the ATA interface but other commands are issued in
P-CHS mode. Because the L-CHS is determined by table lookup
based on total drive capacity and not by a multiply/divide of
the P-CHS cylinder and head values, it may not be possible to
use the simple (and faster) bit shifting L-CHS to P-CHS
algorithms.
When "LBA mode" is disabled, this BIOS looks like a BIOS type
2 with an FDPT. The L-CHS used is taken either from the BIOS
drive type table or from the device's Identify Device data.
This L-CHS can be very different from the L-CHS returned when
"LBA mode" is enabled.
This BIOS may support FDPT/EDPT for up to four drives in the
same manner as described in BIOS type 4.
The basic problem with this BIOS is that the CHS returned by
INT 13H AH=08H changes because of a change in the "LBA mode"
setting in the BIOS setup. This should not happen. This use
or non-use of LBA at the ATA interface should have no effect
on the CHS returned by INT 13H AH=08H. This is the only BIOS
type know to have this problem.
Support issues: If the user changes the "LBA mode" setting in
BIOS setup, INT 13H AH=08H and the FDPT/EDPT change
which may cause *** DATA CORRUPTION ***. The user should be
warned to not change the "LBA mode" setting in BIOS setup once
the drive has been partitioned and software installed.
Different CHS translation algorithms may be used by the BIOS
and an OS.
BIOS Type 8
Origin: Unknown. Perhaps Ontrack's Disk Manager was the
first of these software drivers. Another example of such a
driver is Micro House's EZ Drive.
BIOS call support: Unknown (anyone care to help out here?).
Mostly likely only INT 13H AH=0xH are support. Probably a
FDPT or EDPT exists for drives 80H and 81H.
Description: A software driver that "hides" in the MBR such
that it is loaded into system memory before any OS boot
processing starts. These drivers can have up to three parts:
a part that hides in the MBR, a part that hides in the
remaining sectors of cylinder 0, head 0, and an OS device
driver. The part in the MBR loads the second part of the
driver from cylinder 0 head 0. The second part provides a
replacement for INT 13H that enables CHS translation for at
least the boot drive. Usually the boot drive is defined in
CMOS setup as a type 1 or 2 (5MB or 10MB drive). Once the
second part of the driver is loaded, this definition is
changed to describe the true capacity of the drive and INT 13H
is replaced by the driver's version of INT 13H that does CHS
translation. In some cases the third part, an OS specific
device driver, must be loaded to enable CHS translation for
devices other than the boot device.
I don't know the details of how these drivers respond to INT
13H AH=08H or how they set up drive parameter tables (anyone
care to help out here?). Some of these drivers convert the
L-CHS to an LBA, then they add a small number to the LBA and
finally they convert the LBA to a P-CHS. This in effect skips
over some sectors at the front of the disk.
Support issues: Several identified -- Some OS installation
programs will remove or overlay these drivers; some of these
drivers do not perform CHS translation using the same
algorithms used by the other BIOS types; special OS device
drivers may be required in order to use these software drivers
For example, under MS Windows the standard FastDisk driver
(the 32-bit disk access driver) must be replaced by a driver
that understands the Ontrack, Micro House, etc, version of INT
13H. Different CHS translation algorithms may be used by the
driver and an OS.
The hard disk vendors have been shipping these drivers with
their drives over 528MB during the last year and they have
been ignoring the statements of Microsoft and IBM that these
drivers would not be supported in future OS's. Now it appears
that both Microsoft and IBM are in a panic trying to figure
out how to support some of these drivers in WinNT, Win95 and
OS/2. It is unclear what the outcome of this will be at this
time.
NOTE: THIS IS NOT A PRODUCT ENDORSEMENT! An alternate
solution for an older ISA system is one of the BIOS
replacement cards. This cards have a BIOS option ROM. AMI
makes such a card called the "Disk Extender". This card
replaces the motherboard's INT 13H BIOS with a INT 13H BIOS
that does some form of CHS translation. Another solution for
older VL-Bus systems is an ATA-2 (EIDE) type host adapter card
that provides a option ROM with an INT 13H replacement.
BIOS Type 9
Origin: SCSI host adapters.
BIOS call support: Probably INT 13H AH=0xH and FDPT for BIOS
drives 80H and 81H, perhaps INT 13H AH=4xH.
Description: Most SCSI host adapters contain an option ROM
that enables INT 13 support for the attached SCSI hard drives.
It is possible to have more than one SCSI host adapter, each
with its own option ROM. The CHS used at the INT 13H
interface is converted to the LBA that is used in the SCSI
commands. INT 13H AH=08H returns a CHS. This CHS will have
<=1024 cylinders, <=256 heads and <=63 sectors. The FDPT
probably will exist for SCSI drives with BIOS drive numbers of
80H and 81H and probably indicates the same CHS as that
returned by INT 13H AH=08H. Even though the CHS used at the
INT 13H interface looks like a translated CHS, there is no
need to use a EDPT since there is no CHS-to-CHS translation
used. Other BIOS calls (most likely host adapter specific)
must be used to determine other information about the host
adapter or the drives.
The INT 13H AH=4xH calls can be used to get beyond 8GB but
since there is little support for these calls in today's OS's,
there are probably few SCSI host adapters that support these
newer INT 13H calls.
Support issues: Some SCSI host adapters will not install
their option ROM if there are two INT 13H devices previously
installed by another INT 13H BIOS (for example, two
MFM/RLL/ESDI/ATA drives). Other SCSI adapters will install
their option ROM and use BIOS drive numbers greater than 81H.
Some older OS's don't understand or use BIOS drive numbers
greater than 81H. SCSI adapters are currently faced with the
>8GB drive problem.
BIOS Type 10
Origin: A european system vendor.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H.
Description: This BIOS supports drives >528MB but it does not
support CHS translation. It supports only ATA drives with LBA
capability. INT 13H AH=08H returns an L-CHS. The L-CHS is
converted directly to an LBA. The BIOS sets the ATA drive to
a P-CHS of 16 heads and 63 sectors using the Initialize Drive
Parameters command but it does not use this P-CHS at the ATA
interface.
Support issues: OS/2 will probably work with this BIOS as
long as the drive's power on default P-CHS mode uses 16 heads
and 63 sectors. Because there is no EDPT, OS/2 uses the ATA
Identify Device power on default P-CHS, described in
Identify Device words 1, 3 and 6 as the current P-CHS for the
drive. However, this may not represent the correct P-CHS. A
newer drive will have the its current P-CHS information in
Identify Device words 53-58 but for some reason OS/2 does not
use this information.
/end of part 2 of 2/
NOTICE (18 Dec 99): See the file README.TXT!
How It Works -- CHS Translation
Plus BIOS Types, LBA and Other Good Stuff
Part 1 of 2
Version 4b -- 24 Jun 99
by Hale Landis <hlandis@ibm.net>
NOTE: A "!" at the left margin of the first line of a paragraph
indicates that some change was made in that paragraph.
Introduction (READ THIS!)
-------------------------
This is very technical. Please read carefully. There is lots of
information here that can sound confusing the first time you read
it.
Why is an understanding of how a BIOS works so important? The
basic reason is that the information returned by INT 13H AH=08H
is used by FDISK, it is used in the partition table entries
within a partition record (like the Master Boot Record) that are
created by FDISK, and it is used by the small boot program that
FDISK places into the Master Boot Record. The information
returned by INT 13H AH=08H is in cylinder/head/sector (CHS)
format -- it is not in LBA format. The boot processing done by
your computer's BIOS (INT 19H and INT 13H) is all CHS based.
Read this so that you are not confused by all the false
information going around that says "LBA solves the >528MB
problem".
Read this so that you understand the possible data integrity
problem that a WD EIDE type BIOS creates. Any BIOS that has a
"LBA mode" in the BIOS setup could be a WD EIDE BIOS. Be very
careful and NEVER chage the "LBA mode" setting after you have
partitioned and installed your software.
History
-------
Changes between this version and the preceeding version are
marked by "!" at left margin of the first line of a changed
or new paragraph.
Version 4 -- BIOS Types 8 and 10 updated.
Version 3 -- New BIOS types found and added to this list. More
detailed information is listed for each BIOS type. A section
describing CHS translation was added.
Version 2 -- A rewrite of version 1 adding BIOS types not
included in version 1.
Version 1 -- First attempt to classify the BIOS types and
describe what each does or does not do.
Definitions
-----------
* 528MB - The maximun drive capacity that is supported by 1024
cylinders, 16 heads and 63 sectors (1024x16x63x512). This
is the limit for CHS addressing in the original IBM PC/XT
and IBM PC/AT INT 13H BIOS.
* 8GB - The maximum drive capacity that can be supported by 1024
cylinders, 256 heads and 63 sectors (1024x256x63x512). This
is the limit for the BIOS INT 13H AH=0xH calls.
* ATA - AT Attachment -- The real name of what is widely known
as IDE.
* CE Cylinder - Customer Engineering cylinder. This is the
last cylinder in P-CHS mode. IBM has always reserved this
cylinder for use of disk diagnostic programs. Many BIOS do
not account for it correctly. It is of questionable value
these days and probably should be considered obsolete.
However, since there is no industry wide agreement, beware.
There is no CE Cylinder reserved in the L-CHS address. Also
beware of diagnostic programs that don't realize they are
operating in L-CHS mode and think that the last L-CHS cylinder
is the CE Cylinder.
* CHS - Cylinder/Head/Sector. This is the traditional way to
address sectors on a disk. There are at least two types
of CHS addressing: the CHS that is used at the INT 13H
interface and the CHS that is used at the ATA device
interface. In the MFM/RLL/ESDI and early ATA days the CHS
used at the INT 13H interface was the same as the CHS used at
the device interface.
Today we have CHS translating BIOS types that can use one CHS
at the INT 13H interface and a different CHS at the device
interface. These two types of CHS will be called the logical
CHS or L-CHS and the physical CHS or P-CHS in this document.
L-CHS is the CHS used at the INT 13H interface and P-CHS is
the CHS used at the device interface.
The L-CHS used at the INT 13 interface allows up to 256 heads,
up to 1024 cylinders and up to 63 sectors. This allows
support of up to 8GB drives. This scheme started with either
ESDI or SCSI adapters many years ago.
The P-CHS used at the device interface allows up to 16 heads
up to 65535 cylinders, and up to 63 sectors. This allows
access to 2^28 sectors (136GB) on an ATA device. When a P-CHS
is used at the INT 13H interface it is limited to 1024
cylinders, 16 heads and 63 sectors. This is where the old
528MB limit originated.
ATA devices may also support LBA at the device interface. LBA
allows access to approximately 2^28 sectors (137GB) on an ATA
device.
A SCSI host adapter can convert a L-CHS directly to an LBA
used in the SCSI read/write commands. On a PC today, SCSI is
also limited to 8GB when CHS addressing is used at the INT 13H
interface.
* EDPT - Enhanced fixed Disk Parameter Table -- This table
returns additional information for BIOS drive numbers 80H and
81H. The EDPT for BIOS drive 80H is pointed to by INT 41H.
The EDPT for BIOS drive 81H is pointed to by INT 46H. The
EDPT is a fixed disk parameter table with an AxH signature
byte. This table format returns two sets of CHS information.
One set is the L-CHS and is probably the same as returned by
INT 13H AH=08H. The other set is the P-CHS used at the drive
interface. This type of table allows drives with >1024
cylinders or drives >528MB to be supported. The translated
CHS will have <=1024 cylinders and (probably) >16 heads. The
CHS used at the drive interface will have >1024 cylinders and
<=16 heads. It is unclear how the IBM defined CE cylinder is
accounted for in such a table. Compaq probably gets the
credit for the original definition of this type of table.
* FDPT - Fixed Disk Parameter Table - This table returns
additional information for BIOS drive numbers 80H and 81H.
The FDPT for BIOS drive 80H is pointed to by INT 41H. The
FDPT for BIOS drive 81H is pointed to by INT 46H. A FDPT does
not have a AxH signature byte. This table format returns a
single set of CHS information. The L-CHS information returned
by this table is probably the same as the P-CHS and is also
probably the same as the L-CHS returned by INT 13H AH=08H.
However, not all BIOS properly account for the IBM defined CE
cylinder and this can cause a one or two cylinder difference
between the number of cylinders returned in the AH=08H data
and the FDPT data. IBM gets the credit for the original
definition of this type of table.
* LBA - Logical Block Address. Another way of addressing
sectors that uses a simple numbering scheme starting with zero
as the address of the first sector on a device. The ATA
standard requires that cylinder 0, head 0, sector 1 address
the same sector as addressed by LBA 0. LBA addressing can be
used at the ATA interface if the ATA device supports it. LBA
addressing is also used at the INT 13H interface by the AH=4xH
read/write calls.
* L-CHS -- Logical CHS. The CHS used at the INT 13H interface by
the AH=0xH calls. See CHS above.
* MBR - Master Boot Record (also known as a partition table) -
The sector located at cylinder 0 head 0 sector 1 (or LBA 0).
This sector is created by an "FDISK" utility program. The MBR
may be the only partition table sector or the MBR can be the
first of multiple partition table sectors that form a linked
list. A partition table entry can describe the starting and
ending sector addresses of a partition (also known as a
logical volume or a logical drive) in both L-CHS and LBA form.
Partition table entries use the L-CHS returned by INT 13H
AH=08H. Older FDISK programs may not compute valid LBA
values.
* OS - Operating System.
* P-CHS -- Physical CHS. The CHS used at the ATA device
interface. This CHS is also used at the INT 13H interface by
older BIOS's that do not support >1024 cylinders or >528MB.
See CHS above.
Background and Assumptions
--------------------------
First, please note that this is written with the OS implementor
in mind and that I am talking about the possible BIOS types as
seen by an OS during its hardware configuration search.
It is very important that you not be confused by all the
misinformation going around these days. All OS's that want to be
co-resident with another OS (and that is all of the PC based OS's
that I know of) MUST use INT 13H to determine the capacity of a
hard disk. And that capacity information MUST be determined in
L-CHS mode. Why is this? Because: 1) FDISK and the partition
tables are really L-CHS based, and 2) MS/PC DOS uses INT 13H
AH=02H and AH=03H to read and write the disk and these BIOS calls
are L-CHS based. The boot processing done by the BIOS is all
L-CHS based. During the boot processing, all of the disk read
accesses are done in L-CHS mode via INT 13H and this includes
loading the first of the OS's kernel code or boot manager's code.
Second, because there can be multiple BIOS types in any one
system, each drive may be under the control of a different type
of BIOS. For example, drive 80H (the first hard drive) could be
controlled by the original system BIOS, drive 81H (the second
drive) could be controlled by a option ROM BIOS and drive 82H
(the third drive) could be controlled by a software driver.
Also, be aware that each drive could be a different type, for
example, drive 80H could be an MFM drive, drive 81H could be an
ATA drive, drive 82H could be a SCSI drive.
Third, not all OS's understand or use BIOS drive numbers greater
than 81H. Even if there is INT 13H support for drives 82H or
greater, the OS may not use that support.
Fourth, the BIOS INT 13H configuration calls are:
* AH=08H, Get Drive Parameters -- This call is restricted to
drives up to 528MB without CHS translation and to drives up to
8GB with CHS translation. For older BIOS with no support for
>1024 cylinders or >528MB, this call returns the same CHS as
is used at the ATA interface (the P-CHS). For newer BIOS's
that do support >1024 cylinders or >528MB, this call returns a
translated CHS (the L-CHS). The CHS returned by this call is
used by FDISK to build partition records.
* AH=41H, Get BIOS Extensions Support -- This call is used to
determine if the IBM/Microsoft Extensions or if the Phoenix
Enhanced INT 13H calls are supported for the BIOS drive
number.
* AH=48H, Extended Get Drive Parameters -- This call is used to
determine the CHS geometries, LBA information and other data
about the BIOS drive number.
* the FDPT or EDPT -- While not actually a call, but instead a
data area, the FDPT or EDPT can return additional information
about a drive.
* other tables -- The IBM/Microsoft extensions provide a pointer
to a drive parameter table via INT 13H AH=48H. The Phoenix
enhancement provides a pointer to a drive parameter table
extension via INT 13H AH=48H. These tables are NOT the same
as the FDPT or EDPT.
Note: The INT 13H AH=4xH calls duplicate the older AH=0xH calls
but use a different parameter passing structure. This new
structure allows support of drives with up to 2^64 sectors
(really BIG drives). While at the INT 13H interface the AH=4xH
calls are LBA based, these calls do NOT require that the drive
support LBA addressing.
CHS Translation Algorithms
--------------------------
NOTE: Before you send me email about this, read this entire
section. Thanks!
As you read this, don't forget that all of the boot processing
done by the system BIOS via INT 19H and INT 13H use only the INT
13H AH=0xH calls and that all of this processing is done in CHS
mode.
First, lets review all the different ways a BIOS can be called
to perform read/write operations and the conversions that a BIOS
must support.
* An old BIOS (like BIOS type 1 below) does no CHS translation
and does not use LBA. It only supports the AH=0xH calls:
INT 13H (L-CHS == P-CHS) ATA
AH=0xH --------------------------------> device
(L-CHS) (P-CHS)
* A newer BIOS may support CHS translation and it may support
LBA at the ATA interface:
INT 13H L-CHS ATA
AH=0xH --+--> to --+----------------> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
* A really new BIOS may also support the AH=4xH in addtion to
the older AH\0xH calls. This BIOS must support all possible
combinations of CHS and LBA at both the INT 13H and ATA
interfaces:
INT 13H ATA
AH=4xH --+-----------------------------> device
(LBA) | (LBA)
|
| LBA
+--> to ---------------+
P-CHS |
|
INT 13H L-CHS | ATA
AH=0xH --+--> to --+------------+---> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
You would think there is only one L-CHS to P-CHS translation
algorithm, only one L-CHS to LBA translation algorithm and only
one P-CHS to LBA translation algorithm. But this is not so.
Why? Because there is no document that standardizes such an
algorithm. You can not rely on all BIOS's and OS's to do these
translations the same way.
The following explains what is widely accepted as the
"correct" algorithms.
An ATA disk must implement both CHS and LBA addressing and
must at any given time support only one P-CHS at the device
interface. And, the drive must maintain a strick relationship
between the sector addressing in CHS mode and LBA mode. Quoting
the ATA-2 document:
LBA = ( (cylinder * heads_per_cylinder + heads )
* sectors_per_track ) + sector - 1
where heads_per_cylinder and sectors_per_track are the current
translation mode values.
This algorithm can also be used by a BIOS or an OS to convert
a L-CHS to an LBA as we'll see below.
This algorithm can be reversed such that an LBA can be
converted to a CHS:
cylinder = LBA / (heads_per_cylinder * sectors_per_track)
temp = LBA % (heads_per_cylinder * sectors_per_track)
head = temp / sectors_per_track
sector = temp % sectors_per_track + 1
While most OS's compute disk addresses in an LBA scheme, an OS
like DOS must convert that LBA to a CHS in order to call INT 13H.
Technically an INT 13H should follow this process when
converting an L-CHS to a P-CHS:
1) convert the L-CHS to an LBA,
2) convert the LBA to a P-CHS,
If an LBA is required at the ATA interface, then this third
step is needed:
3) convert the P-CHS to an LBA.
All of these conversions are done by normal arithmetic.
However, while this is the technically correct way to do
things, certain short cuts can be taken. It is possible to
convert an L-CHS directly to a P-CHS using bit a bit shifting
algorithm. This combines steps 1 and 2. And, if the ATA device
being used supports LBA, steps 2 and 3 are not needed. The LBA
value produced in step 1 is the same as the LBA value produced in
step 3.
AN EXAMPLE
Lets look at an example. Lets say that the L-CHS is 1000
cylinders 10 heads and 50 sectors, the P-CHS is 2000 cylinders, 5
heads and 50 sectors. Lets say we want to access the sector at
L-CHS 2,4,3.
* step 1 converts the L-CHS to an LBA,
lba = 1202 = ( ( 2 * 10 + 4 ) * 50 ) + 3 - 1
* step 2 converts the LBA to the P-CHS,
cylinder = 4 = ( 1202 / ( 5 * 50 )
temp = 202 = ( 1202 % ( 5 * 50 ) )
head = 4 = ( 202 / 50 )
sector = 3 = ( 202 % 50 ) + 1
* step 3 converts the P-CHS to an LBA,
lba = 1202 = ( ( 4 * 5 + 4 ) * 50 ) + 3 - 1
Most BIOS (or OS) software is not going to do all of this to
convert an address. Most will use some other algorithm. There
are many such algorithms.
BIT SHIFTING INSTEAD
If the L-CHS is produced from the P-CHS by 1) dividing the
P-CHS cylinders by N, and 2) multiplying the P-CHS heads by N,
where N is 2, 4, 8, ..., then this bit shifting algorithm can be
used and N becomes a bit shift value. This is the most common
way to make the P-CHS geometry of a >528MB drive fit the INT 13H
L-CHS rules. Plus this algorithm maintains the same sector
ordering as the more complex algorithm above. Note the
following:
Lcylinder = L-CHS cylinder being accessed
Lhead = L-CHS head being accessed
Lsector = L-CHS sector being accessed
Pcylinder = the P-CHS cylinder being accessed
Phead = the P-CHS head being accessed
Psector = P-CHS sector being accessed
NPH = is the number of heads in the P-CHS
N = 2, 4, 8, ..., the bit shift value
The algorithm, which can be implemented using bit shifting
instead of multiply and divide operations is:
Pcylinder = ( Lcylinder * N ) + ( Lhead / NPH );
Phead = ( Lhead % NPH );
Psector = Lsector;
A BIT SHIFTING EXAMPLE
Lets apply this to our example above (L-CHS = 1000,10,50 and
P-CHS = 2000, 5, 50) and access the same sector at at L-CHS
2,4,3.
Pcylinder = 4 = ( 2 * 2 ) + ( 4 / 5 )
Phead = 4 = ( 4 % 5 )
Psector = 3 = 3
As you can see, this produces the same P-CHS as the more
complex method above.
SO WHAT IS THE PROBLEM?
The basic problem is that there is no requirement that a CHS
translating BIOS followed these rules. There are many other
algorithms that can be implemented to perform a similar function.
Today, there are at least two popular implementions: the Phoenix
implementation (described above) and the non-Phoenix
implementations.
SO WHY IS THIS A PROBLEM IF IT IS HIDDEN INSIDE THE BIOS?
Because a protected mode OS that does not want to use INT 13H
must implement the same CHS translation algorithm. If it
doesn't, your data gets scrambled.
WHY USE CHS AT ALL?
In the perfect world of tomorrow, maybe only LBA will be used.
But today we are faced with the following problems:
* Some drives >528MB don't implement LBA.
* Some drives are optimized for CHS and may have lower
performance when given commands in LBA mode. Don't forget
that LBA is something new for the ATA disk designers who have
worked very hard for many years to optimize CHS address
handling. And not all drive designs require the use of LBA
internally.
* The L-CHS to LBA conversion is more complex and slower than
the bit shifting L-CHS to P-CHS conversion.
* DOS, FDISK and the MBR are still CHS based -- they use the
CHS returned by INT 13H AH=08H. Any OS that can be installed
on the same disk with DOS must understand CHS addressing.
* The BIOS boot processing and loading of the first OS kernel
code is done in CHS mode -- the CHS returned by INT 13H AH=08H
is used.
* Microsoft has said that their OS's will not use any disk
capacity that can not also be accessed by INT 13H AH=0xH.
These are difficult problems to overcome in today's industry
environment. The result: chaos.
DANGER TO YOUR DATA!
See the description of BIOS Type 7 below to understand why a
WD EIDE BIOS is so dangerous to your data.
! AN INTERESTING TABLE
The following table was created and used by T13 when discussing
the various CHS and LBA problems in x86 PC BIOS. You may find it
interesting.
In many rows, both the drive's CHS and the resulting INT 13
translated CHS are shown. Any CHS with more than 16 heads is a
translated CHS.
-----------------------------------------------------------------
Number of Approx
CHS Sectors Bytes Note
------------- ----------- ------- -------------------------
1024 16 63 1,032,192 528.4MB Max w/o CHS translation.
8192 16 63 8,257,536 4.228GB This is the 15/16 head
1024 128 63 8,257,536 4,228GB boundary (see note 3).
16383 15 63 15,481,935 7.926GB Bit-shifting
1023 240 63 15,467,760 7.919GB BIOS max.
16384 15 63 15,482,880 7.927GB What a
1024 240 63 15,482,880 7.927GB nice fit.
1024 255 63 16,450,560 8.423GB LBA assisted BIOS max.
16383 16 63 16,514,064 8,455GB ID word 1, 3, 6 limit
(per ATA annex).
16384 16 63 16,515,072 8.456GB Another nice fit but
1024 256 63 16,515,072 8.456GB we can't use it.
32768 16 63 33,030,144 16.91GB Valid drive CHS but no
valid BIOS translation
65536 16 63 66,060,288 33.82GB Valid drive CHS but no
valid BIOS translation.
65535 16 255 267,382,800 136.9GB CHS max drive size
in ID words.
65536 16 255 267,386,880 136.9GB CHS max drive size
addressable.
28-bit lba 268,435,456 137.4GB LBA max drive size.
-----------------------------------------------------------------
Notes:
1) A drive really can't have 65536 cylinders, numbered 0 to 65535
(or FFFFH). The max number of cylinders is 65535, numbered 0 to
65534 (or FFFEH). This is because the maximum value in ID words
1 and 54 is 65535 (or FFFFH). But beware that a media access
command could attempt to access cylinder number 65535 (or FFFFH).
2) LBA is able to address about 0.5GB more than CHS because there
is no sector number 0 in CHS.
3) A drive that reports 16 heads in ID word 3 and more than 8192
cylinders in ID word 1 will cause a bit-shifting BIOS to use a
CHS translation with 256 heads. Some operating systems save only
the lower 8-bits of this value resulting in zero heads. Many
bit-shitting BIOS will IDP such a drive to 15 heads, so that the
translated CHS is 240 heads, in order to avoid this problem.
/end of part 1 of 2 -- continued in part 2 of 2/
NOTICE (18 Dec 99): See the file README.TXT!
How It Works -- DOS Floppy Disk Boot Sector
Version 1a
by Hale Landis <hlandis@ibm.net>
DOS FLOPPY DISK BOOT SECTOR
This article is a disassembly of a floppy disk boot sector for a
DOS floppy. The boot sector of a floppy disk is located at
cylinder 0, head 0, sector 1. This sector is created by a floppy
disk formating program, such as the DOS FORMAT program. The boot
sector of a FAT hard disk partition has a similar layout and
function. Basically a bootable FAT hard disk partition looks
like a big floppy during the early stages of the system's boot
processing.
At the completion of your system's Power On Self Test (POST), INT
19 is called. Usually INT 19 tries to read a boot sector from
the first floppy drive. If a boot sector is found on the floppy
disk, the that boot sector is read into memory at location
0000:7C00 and INT 19 jumps to memory location 0000:7C00.
However, if no boot sector is found on the first floppy drive,
INT 19 tries to read the MBR from the first hard drive. If an
MBR is found it is read into memory at location 0000:7c00 and INT
19 jumps to memory location 0000:7c00. The small program in the
MBR will attempt to locate an active (bootable) partition in its
partition table. If such a partition is found, the boot sector
of that partition is read into memory at location 0000:7C00 and
the MBR program jumps to memory location 0000:7C00. Each
operating system has its own boot sector format. The small
program in the boot sector must locate the first part of the
operating system's kernel loader program (or perhaps the kernel
itself or perhaps a "boot manager program") and read that into
memory.
INT 19 is also called when the CTRL-ALT-DEL keys are used. On
most systems, CTRL-ALT-DEL causes an short version of the POST to
be executed before INT 19 is called.
=====
Where stuff is:
The BIOS Parameter Block (BPB) starts at offset 0.
The boot sector program starts at offset 3e.
The messages issued by this program start at offset 19e.
The DOS hidden file names start at offset 1e6.
The boot sector signature is at offset 1fe.
Here is a summary of what this thing does:
1) Copy Diskette Parameter Table which is pointed to by INT 1E.
2) Alter the copy of the Diskette Parameter Table.
3) Alter INT 1E to point to altered Diskette Parameter Table.
4) Do INT 13 AH=00, disk reset call.
5) Compute sector address of root directory.
6) Read first sector of root directory into 0000:0500.
7) Confirm that first two directory entries are for IO.SYS
and MSDOS.SYS.
8) Read first 3 sectors of IO.SYS into 0000:0700 (or 0070:0000).
9) Leave some information in the registers and jump to
IO.SYS at 0070:0000.
NOTE:
This program uses the CHS based INT 13H AH=02 to read the FAT
root directory and to read the IO.SYS file. If the drive is
>528MB, this CHS must be a translated CHS (or L-CHS, see my
BIOS TYPES document). Except for internal computations no
addresses in LBA form are used, another reason why LBA doesn't
solve the >528MB problem.
=====
Here is the entire sector in hex and ascii.
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 eb3c904d 53444f53 352e3000 02010100 *.<.MSDOS5.0.....*
000010 02e00040 0bf00900 12000200 00000000 *...@............*
000020 00000000 0000295a 5418264e 4f204e41 *......)ZT.&NO NA*
000030 4d452020 20204641 54313220 2020fa33 *ME FAT12 .3*
000040 c08ed0bc 007c1607 bb780036 c5371e56 *.....|...x.6.7.V*
000050 1653bf3e 7cb90b00 fcf3a406 1fc645fe *.S.>|.........E.*
000060 0f8b0e18 7c884df9 894702c7 073e7cfb *....|.M..G...>|.*
000070 cd137279 33c03906 137c7408 8b0e137c *..ry3.9..|t....|*
000080 890e207c a0107cf7 26167c03 061c7c13 *.. |..|.&.|...|.*
000090 161e7c03 060e7c83 d200a350 7c891652 *..|...|....P|..R*
0000a0 7ca3497c 89164b7c b82000f7 26117c8b *|.I|..K|. ..&.|.*
0000b0 1e0b7c03 c348f7f3 0106497c 83164b7c *..|..H....I|..K|*
0000c0 00bb0005 8b16527c a1507ce8 9200721d *......R|.P|...r.*
0000d0 b001e8ac 0072168b fbb90b00 bee67df3 *.....r........}.*
0000e0 a6750a8d 7f20b90b 00f3a674 18be9e7d *.u... .....t...}*
0000f0 e85f0033 c0cd165e 1f8f048f 4402cd19 *._.3...^....D...*
000100 585858eb e88b471a 48488a1e 0d7c32ff *XXX...G.HH...|2.*
000110 f7e30306 497c1316 4b7cbb00 07b90300 *....I|..K|......*
000120 505251e8 3a0072d8 b001e854 00595a58 *PRQ.:.r....T.YZX*
000130 72bb0501 0083d200 031e0b7c e2e28a2e *r..........|....*
000140 157c8a16 247c8b1e 497ca14b 7cea0000 *.|..$|..I|.K|...*
000150 7000ac0a c07429b4 0ebb0700 cd10ebf2 *p....t).........*
000160 3b16187c 7319f736 187cfec2 88164f7c *;..|s..6.|....O|*
000170 33d2f736 1a7c8816 257ca34d 7cf8c3f9 *3..6.|..%|.M|...*
000180 c3b4028b 164d7cb1 06d2e60a 364f7c8b *.....M|.....6O|.*
000190 ca86e98a 16247c8a 36257ccd 13c30d0a *.....$|.6%|.....*
0001a0 4e6f6e2d 53797374 656d2064 69736b20 *Non-System disk *
0001b0 6f722064 69736b20 6572726f 720d0a52 *or disk error..R*
0001c0 65706c61 63652061 6e642070 72657373 *eplace and press*
0001d0 20616e79 206b6579 20776865 6e207265 * any key when re*
0001e0 6164790d 0a00494f 20202020 20205359 *ady...IO SY*
0001f0 534d5344 4f532020 20535953 000055aa *SMSDOS SYS..U.*
=====
The first 62 bytes of a boot sector are known as the BIOS
Parameter Block (BPB). Here is the layout of the BPB fields
and the values they are assigned in this boot sector:
db JMP instruction at 7c00 size 2 = eb3c
db NOP instruction 7c02 1 90
db OEMname 7c03 8 'MSDOS5.0'
dw bytesPerSector 7c0b 2 0200
db sectPerCluster 7c0d 1 01
dw reservedSectors 7c0e 2 0001
db numFAT 7c10 1 02
dw numRootDirEntries 7c11 2 00e0
dw numSectors 7c13 2 0b40 (ignore numSectorsHuge)
db mediaType 7c15 1 f0
dw numFATsectors 7c16 2 0009
dw sectorsPerTrack 7c18 2 0012
dw numHeads 7c1a 2 0002
dd numHiddenSectors 7c1c 4 00000000
dd numSectorsHuge 7c20 4 00000000
db driveNum 7c24 1 00
db reserved 7c25 1 00
db signature 7c26 1 29
dd volumeID 7c27 4 5a541826
db volumeLabel 7c2b 11 'NO NAME '
db fileSysType 7c36 8 'FAT12 '
=====
Here is the boot sector...
The first 3 bytes of the BPB are JMP and NOP instructions.
0000:7C00 EB3C JMP START
0000:7C02 90 NOP
Here is the rest of the BPB.
0000:7C00 ......4d 53444f53 352e3000 02010100 * MSDOS5.0.....*
0000:7C10 02e00040 0bf00900 12000200 00000000 *...@............*
0000:7C20 00000000 0000295a 5418264e 4f204e41 *......)ZT.&NO NA*
0000:7C30 4d452020 20204641 54313220 2020.... *ME FAT12 *
Now pay attention here...
The 11 bytes starting at 0000:7c3e are immediately overlaid by
information copied from another part of memory. That
information is the Diskette Parameter Table. This data is
pointed to by INT 1E. This data is:
7c3e = Step rate and head unload time.
7c3f = Head load time and DMA mode flag.
7c40 = Delay for motor turn off.
7c41 = Bytes per sector.
7c42 = Sectors per track.
7c43 = Intersector gap length.
7c44 = Data length.
7c45 = Intersector gap length during format.
7c46 = Format byte value.
7c47 = Head settling time.
7c48 = Delay until motor at normal speed.
The 11 bytes starting at 0000:7c49 are also overlaid by the
following data:
7c49 - 7c4c = diskette sector address (as LBA)
of the data area.
7c4d - 7c4e = cylinder number to read from.
7c4f - 7c4f = sector number to read from.
7c50 - 7c53 = diskette sector address (as LBA)
of the root directory.
START: START OF BOOT SECTOR PROGRAM
0000:7C3E FA CLI interrupts off
0000:7C3F 33C0 XOR AX,AX set AX to zero
0000:7C41 8ED0 MOV SS,AX SS is now zero
0000:7C43 BC007C MOV SP,7C00 SP is now 7c00
0000:7C46 16 PUSH SS also set ES
0000:7C47 07 POP ES to zero
The INT 1E vector is at 0000:0078.
Get the address that the vector points to
into the DS:SI registers.
0000:7C48 BB7800 MOV BX,0078 BX is now 78
0000:7C4B 36 SS:
0000:7C4C C537 LDS SI,[BX] DS:SI is now [0:78]
0000:7C4E 1E PUSH DS save DS:SI --
0000:7C4F 56 PUSH SI saves param tbl addr
0000:7C50 16 PUSH SS save SS:BX --
0000:7C51 53 PUSH BX saves INT 1E address
Move the diskette param table to 0000:7c3e.
0000:7C52 BF3E7C MOV DI,7C3E DI is address of START
0000:7C55 B90B00 MOV CX,000B count is 11
0000:7C58 FC CLD clear direction
0000:7C59 F3 REPZ move the diskette param
0000:7C5A A4 MOVSB table to 0000:7c3e
0000:7C5B 06 PUSH ES also set DS
0000:7C5C 1F POP DS to zero
Alter some of the diskette param table data.
0000:7C5D C645FE0F MOV BYTE PTR [DI-02],0F change head settle time
at 0000:7c47
0000:7C61 8B0E187C MOV CX,[7C18] sectors per track
0000:7C65 884DF9 MOV [DI-07],CL save at 0000:7c42
Change INT 1E so that it points to the
altered Diskette param table at 0000:7c3e.
0000:7C68 894702 MOV [BX+02],AX change INT 1E segment
0000:7C6B C7073E7C MOV WORD PTR [BX],7C3E change INT 1E offset
Call INT 13 with AX=0000, disk reset, so
that the new diskette param table is used.
0000:7C6F FB STI interrupts on
0000:7C70 CD13 INT 13 do diskette reset call
0000:7C72 7279 JB TALK jmp if any error
Detemine the starting sector address of
the root directory as an LBA.
0000:7C74 33C0 XOR AX,AX AX is now zero
0000:7C76 3906137C CMP [7C13],AX number sectros zero?
0000:7C7A 7408 JZ SMALL_DISK yes
0000:7C7C 8B0E137C MOV CX,[7C13] number of sectors
0000:7C80 890E207C MOV [7C20],CX save in huge num sects
SMALL_DISK:
0000:7C84 A0107C MOV AL,[7C10] number of FAT tables
0000:7C87 F726167C MUL WORD PTR [7C16] number of fat sectors
0000:7C8B 03061C7C ADD AX,[7C1C] number of hidden sectors
0000:7C8F 13161E7C ADC DX,[7C1E] number of hidden sectors
0000:7C93 03060E7C ADD AX,[7C0E] number of reserved sectors
0000:7C97 83D200 ADC DX,+00 number of reserved sectors
0000:7C9A A3507C MOV [7C50],AX save start addr
0000:7C9D 8916527C MOV [7C52],DX of root dir (as LBA)
0000:7CA1 A3497C MOV [7C49],AX save start addr
0000:7CA4 89164B7C MOV [7C4B],DX of root dir (as LBA)
Determine sector address of first sector
in the data area as an LBA.
0000:7CA8 B82000 MOV AX,0020 size of a dir entry (32)
0000:7CAB F726117C MUL WORD PTR [7C11] number of root dir entries
0000:7CAF 8B1E0B7C MOV BX,[7C0B] bytes per sector
0000:7CB3 03C3 ADD AX,BX
0000:7CB5 48 DEC AX
0000:7CB6 F7F3 DIV BX
0000:7CB8 0106497C ADD [7C49],AX add to start addr
0000:7CBC 83164B7C00 ADC WORD PTR [7C4B],+00 of root dir (as LBA)
Read the first root dir sector into 0000:0500.
0000:7CC1 BB0005 MOV BX,0500 addr to read into
0000:7CC4 8B16527C MOV DX,[7C52] get start of address
0000:7CC8 A1507C MOV AX,[7C50] of root dir (as LBA)
0000:7CCB E89200 CALL CONVERT call conversion routine
0000:7CCE 721D JB TALK jmp is any error
0000:7CD0 B001 MOV AL,01 read 1 sector
0000:7CD2 E8AC00 CALL READ_SECTORS read 1st root dir sector
0000:7CD5 7216 JB TALK jmp if any error
0000:7CD7 8BFB MOV DI,BX addr of 1st dir entry
0000:7CD9 B90B00 MOV CX,000B count is 11
0000:7CDC BEE67D MOV SI,7DE6 addr of file names
0000:7CDF F3 REPZ is this "IO.SYS"?
0000:7CE0 A6 CMPSB
0000:7CE1 750A JNZ TALK no
0000:7CE3 8D7F20 LEA DI,[BX+20] addr of next dir entry
0000:7CE6 B90B00 MOV CX,000B count is 11
0000:7CE9 F3 REPZ is this "MSDOS.SYS"?
0000:7CEA A6 CMPSB
0000:7CEB 7418 JZ FOUND_FILES they are equal
TALK:
Display "Non-System disk..." message,
wait for user to hit a key, restore
the INT 1E vector and then
call INT 19 to start boot processing
all over again.
0000:7CED BE9E7D MOV SI,7D9E "Non-System disk..."
0000:7CF0 E85F00 CALL MSG_LOOP display message
0000:7CF3 33C0 XOR AX,AX INT 16 function
0000:7CF5 CD16 INT 16 read keyboard
0000:7CF7 5E POP SI get INT 1E vector's
0000:7CF8 1F POP DS address
0000:7CF9 8F04 POP [SI] restore the INT 1E
0000:7CFB 8F4402 POP [SI+02] vector's data
0000:7CFE CD19 INT 19 CALL INT 19 to try again
SETUP_TALK:
0000:7D00 58 POP AX pop junk off stack
0000:7D01 58 POP AX pop junk off stack
0000:7D02 58 POP AX pop junk off stack
0000:7D03 EBE8 JMP TALK now talk to the user
FOUND_FILES:
Compute the sector address of the first
sector of IO.SYS.
0000:7D05 8B471A MOV AX,[BX+1A] get starting cluster num
0000:7D08 48 DEC AX subtract 1
0000:7D09 48 DEC AX subtract 1
0000:7D0A 8A1E0D7C MOV BL,[7C0D] sectors per cluster
0000:7D0E 32FF XOR BH,BH
0000:7D10 F7E3 MUL BX multiply
0000:7D12 0306497C ADD AX,[7C49] add start addr of
0000:7D16 13164B7C ADC DX,[7C4B] root dir (as LBA)
Read IO.SYS into memory at 0000:0700. IO.SYS
is 3 sectors long.
0000:7D1A BB0007 MOV BX,0700 address to read into
0000:7D1D B90300 MOV CX,0003 read 3 sectors
READ_LOOP:
Read the first 3 sectors of IO.SYS
(IO.SYS is much longer than 3 sectors).
0000:7D20 50 PUSH AX save AX
0000:7D21 52 PUSH DX save DX
0000:7D22 51 PUSH CX save CX
0000:7D23 E83A00 CALL CONVERT call conversion routine
0000:7D26 72D8 JB SETUP_TALK jmp if error
0000:7D28 B001 MOV AL,01 read one sector
0000:7D2A E85400 CALL READ_SECTORS read one sector
0000:7D2D 59 POP CX restore CX
0000:7D2E 5A POP DX restore DX
0000:7D2F 58 POP AX restore AX
0000:7D30 72BB JB TALK jmp if any INT 13 error
0000:7D32 050100 ADD AX,0001 add one to the sector addr
0000:7D35 83D200 ADC DX,+00 add one to the sector addr
0000:7D38 031E0B7C ADD BX,[7C0B] incr mem addr by sect size
0000:7D3C E2E2 LOOP READ_LOOP read next sector
Leave information in the AX, BX, CX and DX
registers for IO.SYS to use. Finally,
jump to IO.SYS at 0070:0000.
0000:7D3E 8A2E157C MOV CH,[7C15] media type
0000:7D42 8A16247C MOV DL,[7C24] drive number
0000:7D46 8B1E497C MOV BX,[7C49] get start addr of
0000:7D4A A14B7C MOV AX,[7C4B] root dir (as LBA)
0000:7D4D EA00007000 JMP 0070:0000 JUMP TO 0070:0000
MSG_LOOP:
This routine displays a message using
INT 10 one character at a time.
The message address is in DS:SI.
0000:7D52 AC LODSB get message character
0000:7D53 0AC0 OR AL,AL end of message?
0000:7D55 7429 JZ RETURN jmp if yes
0000:7D57 B40E MOV AH,0E display one character
0000:7D59 BB0700 MOV BX,0007 video attrbiutes
0000:7D5C CD10 INT 10 display one character
0000:7D5E EBF2 JMP MSG_LOOP do again
CONVERT:
This routine
converts a sector address (an LBA) to
a CHS address. The LBA is in DX:AX.
0000:7D60 3B16187C CMP DX,[7C18] hi part of LBA > sectPerTrk?
0000:7D64 7319 JNB SET_CARRY jmp if yes
0000:7D66 F736187C DIV WORD PTR [7C18] div by sectors per track
0000:7D6A FEC2 INC DL add 1 to sector number
0000:7D6C 88164F7C MOV [7C4F],DL save sector number
0000:7D70 33D2 XOR DX,DX zero DX
0000:7D72 F7361A7C DIV WORD PTR [7C1A] div number of heads
0000:7D76 8816257C MOV [7C25],DL save head number
0000:7D7A A34D7C MOV [7C4D],AX save cylinder number
0000:7D7D F8 CLC clear carry
0000:7D7E C3 RET return
SET_CARRY:
0000:7D7F F9 STC set carry
RETURN:
0000:7D80 C3 RET return
READ_SECTORS:
The caller of this routine supplies:
AL = number of sectors to read
ES:BX = memory location to read into
and CHS address to read from in
memory locations 7c25 and 7C4d-7c4f.
0000:7D81 B402 MOV AH,02 INT 13 read sectors
0000:7D83 8B164D7C MOV DX,[7C4D] get cylinder number
0000:7D87 B106 MOV CL,06 shift count
0000:7D89 D2E6 SHL DH,CL shift upper cyl left 6 bits
0000:7D8B 0A364F7C OR DH,[7C4F] or in sector number
0000:7D8F 8BCA MOV CX,DX move to CX
0000:7D91 86E9 XCHG CH,CL CH=cyl lo, CL=cyl hi + sect
0000:7D93 8A16247C MOV DL,[7C24] drive number
0000:7D97 8A36257C MOV DH,[7C25] head number
0000:7D9B CD13 INT 13 read sectors
0000:7D9D C3 RET return
Data not used.
0000:7D90 ca86e98a 16247c8a 36257ccd 13c3.... *.....$|.6%|... *
Messages here.
0000:7D90 ........ ........ ........ ....0d0a * ..*
0000:7Da0 4e6f6e2d 53797374 656d2064 69736b20 *Non-System disk *
0000:7Db0 6f722064 69736b20 6572726f 720d0a52 *or disk error..R*
0000:7Dc0 65706c61 63652061 6e642070 72657373 *eplace and press*
0000:7Dd0 20616e79 206b6579 20776865 6e207265 * any key when re*
0000:7De0 6164790d 0a00.... ........ ........ *ady... *
MS DOS hidden file names (first two root directory entries).
0000:7De0 ........ ....494f 20202020 20205359 * IO SY*
0000:7Df0 534d5344 4f532020 20535953 000055aa *SMSDOS SYS..U.*
The last two bytes contain a 55AAH signature.
0000:7Df0 ........ ........ ........ ....55aa * U.*
/end/
NOTICE (18 Dec 99): See the file README.TXT!
ATA/ATA-1/ATA-2/IDE/EIDE/etc FAQ
Part 1 of ? -- The Basics
Version 0b -- 7 Feb 95
by Hale Landis -- hlandis@ibm.net
Note: Major changes from the previous version are marked by a
"!" at the left margin on the first line of the changed
paragraph.
First the "legal" stuff...
1) This FAQ is not intended to replace any other FAQ on this
subject but is an attempt to provide historical and technical
information about the ATA interface.
2) This FAQ is not an endorsement of any vendor's product(s).
3) This FAQ is not a recommendation to purchase any vendor's
product(s).
4) Every effort is made to insure that all of the information
presented here is not copyrighted, not proprietary and
unrestricted.
4) When opinions are stated they are clearly identified,
including the person's name and email address. Such opinions
are offered as long as they contribute to the understanding of
the subject being discussed. No "flames" allowed.
This is the first version of this FAQ. It will take some time to
get all the significant information into it so it will be rapidly
growing and changing during the next several weeks or months. I
don't even know how many parts there will be yet! Versions will
be numbered with simple integer numbers (no 1.1, 1.2, etc)
starting at 0.
If you have a question that is not answered here or if you have
unrestricted material that you would like to contribute, please
email it to hlandis@ibm.net. DO NOT send material that is
copyrighted, proprietary or otherwise restricted in any way -- I
can't use such material in this document.
Table of Contents
-----------------
Part 1 - The Basics
Glossary
Basic Questions
Part 2 - BIOS and Drivers
TBD
Glossary
--------
Read and understand these terms. You will be lost and confused
if you don't! Many of these are describe in much greater detail
in other parts of this FAQ.
ATA or AT Attachment
ATA is the proper and correct name for what most people call
IDE. In this document, ATA refers to all forms of ATA (ATA-1,
ATA-2, etc, IDE, EIDE, etc). The ATA interface uses a single
40-conductor cable in most desktop systems.
ATA-1
ATA-1 is the common name of the original ATA (IDE)
specification. ATA-1 is not an official standard yet. Final
approval is pending.
ATA-2 or ATA Extensions
ATA-2 is the common name of the new ATA specification. ATA-2
is still in early draft form and has not been submitted for
approval as an official standard.
ATA-3
! ATA-3 is the common name of a future version of the ATA
specification. The ATA-3 working group has held several
meeting but the only things adopted so far are a DMA
version of the Identify command, a description of
"device 1 only configurations" and a set of "security"
commands.
! There is much discussion going on concerning merging ATA-3
with ATAPI. This will require some kind of "command overlap"
capability. The details of this are consumming much of the
meeting time.
ATAPI or ATA Packet Interface
ATAPI is a proposed new interface specification. Initially it
will probably be used for CD-ROM and tape devices. It uses
the ATA hardware interface at the physical level but uses a
subset of the SCSI command set at the logical level. The
ATAPI specification work is currently being done in the SFF
committee.
! The ATAPI folks have delayed forwarding their CD-ROM
specification from SFF to X3T10 so the X3T10 ATAPI working
group has nothing to work on yet and have held no meetings.
! Block Mode
! Block mode is the name given to the use of the ATA Read
Multiple and Write Multiple commands. These commands generate
a single interrupt to the host system for each block of
sectors transfered. The traditional Read Sectors and Write
Sectors commands generate an interrupt to the host for each
sector transfered.
CAM (Common Access Method) Committee
The Common Access Method committee, now disbanded, worked on
two specifications: the CAM SCSI and CAM ATA specifications.
Both specifications were forwarded to the X3T9 committee for
further work years ago.
CHS or Cylinder/Head/Sector
CHS is the old and traditional way to address data sectors on
a hard disk. This style of addressing relates a sector's
address to the position of the read/write heads. In today's
ATA devices, all sector addresses used by the host are logical
and have nothing to do with the actual physical position of
the sector on the media or the actual position of the
read/write heads.
Command Block
Control Block
These are names given to the I/O register interface used by
ATA devices. It refers to a set of I/O registers, or I/O
ports and I/O port addresses used to program the device.
These names replace the older term Task File.
DMA or Direct Memory Access
DMA is a method of data transfer between two devices that does
not use the system's main processor as part of the data path.
DMA requires lots of hardware: a DMA arbitration unit, a DMA
data transfer unit and host bus signals that enable the DMA
controller to assume control of the host system's bus. When
the DMA controller has control of the host system's bus, it
moves data between the two devices by generating the
appropriate bus read/write cycles. For the ATA READ DMA
command this means generating an I/O read cycle and then a
memory write cycle for each 16-bit word being transferred.
For the ATA WRITE DMA command, a memory read cycle is followed
by an I/O write cycle for each 16-bit word transferred.
EIDE or Enhanced IDE
EIDE is a marketing program started by Western Digital to
promote certain ATA-2 features including ATAPI. WD has
encouraged other product vendors to mark their products as
"EIDE compatible" or "EIDE capable".
ESDI
See MFM.
Fast ATA
Fast ATA is a Seagate marketing program used to promote
certain ATA-2 features in newer ATA devices. Seagate has
encouraged other product vendors to mark their products as
"Fast ATA compatible" or "Fast ATA capable".
Host or Host System
The computer system that the ATA device is attached to.
HBA or Host Bus Adapter or Host Adapter
The hardware that converts host bus signals to/from ATA
interface signals. An ATA-1 host adapter is generally a very
simple piece of hardware. An ATA-2 host adapter can be simple
or complex.
IDE
IDE can mean any number of things: Imbedded Device (or Drive)
Electronics (yes, you can spell embedded with an "i"),
Intelligent Device (or Drive) Electronics, etc. The term IDE
is the trademark of someone (Western Digital does not claim
IDE as theirs but they do claim EIDE). Many hard disk vendors
do not use IDE to describe their products to avoid any
trademark conflicts.
LBA or Logical Block Addressing
LBA is a newer (for ATA it is newer) way to address data
sectors on a hard disc. This style of addressing uses a
28-bit binary number to address a sector. LBA numbers start
at zero. In today's ATA devices, all sector addresses used by
the host are logical and have nothing to do with the actual
physical location of the sector on the media.
Local Bus
Usually this refers to the processor's local bus in a high
performance computer system. Usually the processor, the
external processor instruction/data cache, the main memory
controller and the bridge controller for the next low speed
system bus, for example, a PCI bus, are located on the local
bus. Lower speed local buses may have connectors that allow
the attachment of other devices. For example, the VL-Bus is a
local bus that can allow attachment of video, SCSI or ATA
controllers. It is very difficult to attach other devices to
high speed (say faster than 100MHz) local buses due to
electrical restrictions that come into play at those higher
speeds.
Master
ATA device 0. Device 0, the master, is the "master" of
nothing. See Slave.
Megabyte or MB
Megabyte or MB is 1,000,000 bytes or 10^6 bytes. IT IS NOT
1,048,576 bytes or 2^20 bytes, repeat NOT!
MFM
In this document MFM refers to any of the older hard disk
controller interfaces, MFM, RLL and ESDI. It is used to
describe any hard disk controller that uses the Task File
interface on the host side and the ST506/ST412 interface
on the drive side.
OS
Operating System.
PC Card ATA
PCMCIA
We can thank the Personal Computer Memory Card International
Association for the PC Card specification. The PCMCIA is a
nonprofit industry association. The PC Card ATA
specification is another form of the ATA interface used by
PCMCIA compatible ATA devices. This interface uses the PCMCIA
68-pin connector. Most 68-pin ATA devices are dual mode --
they can operate as either a PCMCIA PC Card ATA device or as a
68-pin ATA device.
PCI
We can thank Intel and the other members of the PCI committee
for the PCI bus specification. PCI is intended to be the next
high performance computer bus. PCI is not generally described
as a processor local bus.
PIO or Programmed Input/Output
PIO is a method of data transfer between two devices that uses
the system's main processor as part of the data path. On
x86 systems, the REP INS and REP OUT instructions
implement this data transfer method. INS reads and I/O port
and writes the data into memory. OUTS reads data from memory
and writes the data to an I/O port. Each time an INS or OUTS
instruction is executed, the memory address is updated. The
REP prefix causes the instructions to be repeated until a
counter reaches zero.
RLL
See MFM.
Slave
ATA device 1. Device 1, the slave, is a "slave" to nothing.
See Master.
Task File
This is the name given to the I/O register interface used by
MFM controllers. It refers to a set of I/O registers, or I/O
ports and I/O port addresses used to program the controller.
In ATA, this name has been replaced by the terms Command Block
and Control Block.
SCSI
See the SCSI FAQ.
SFF or Small Form Factor
The SFF committee is an ad hoc committee formed by most of the
major storage device and system vendors to set standards for
the physical layout of hard disk and other devices. SFF has
published many specifications that describe the physical
mounting and connector specifications for hard disk devices,
including ATA devices. During a brief period of time when the
X3T9 committee was not doing much work on the ATA-1 interface,
the SFF committee published several specifications that were
not really part of the original SFF charter. Most, if not
all, of these nonphysical specifications have now been
incorporated into the latest X3T9 or X3T10 ATA specifications.
ATAPI is currently an SFF specification.
ST506 and ST412
This is the common name for the hard disk controller to hard
disk drive interface used by MFM, RLL and ESDI controllers and
disk drives. ST stands for Seagate Technology. The ST506 and
ST412 were the Seagate products that set the de facto
standards for this interface many years ago. This interface
is composed of two cables: a 34-conductor and a 20-conductor
cable.
VESA and VL-Bus
We can thank the Video Electronics Standards Association for
the VESA Local Bus or VL-Bus specification. The VL-Bus is one
example of a local bus. VESA is a nonprofit industry
association like the PCMCIA.
WG or Working Group
The actual work on various specifications and standards
documents within the X3T9, X3T10 and SFF committees happens in
working group meetings. Most WG meetings are held monthly.
X3T9 and X3T10
These are the names of the official standards committees that
have worked on the ATA-1 and ATA-2 specifications. X3T9 was
responsible for the SCSI and ATA-1 specifications and
standards. X3T10 has replaced X3T9 and is now responsible for
the current SCSI and ATA specifications and standards work.
528MB
This term is used in this document to describe the capacity
boundary that exists in most x86 system software. This
boundary limits the size of an ATA disk drive to 528MB. For
cylinder/head/sector style addressing of disk data sectors,
this number is computed as follows:
a) the number of cylinders are limited to 1024, numbered
0-1023.
b) the number of heads (per cylinder) are limited to 16,
numbered 0-15,
c) the number of sectors (per track) are limited to 63,
numbered 1-63.
d) a sector is 512 bytes,
e) 528MB means the following values:
( 1024 * 16 * 63 ) or 1,032,192 data sectors
or
( 1024 * 16 * 63 * 512 ) or 528,482,304 bytes.
68-pin ATA
This refers to a variation of the ATA interface that uses the
PCMCIA 68-pin physical interface but does not use the PCMCIA
electrical or logical interface. Most 68-pin ATA devices are
dual mode -- they can operate as either a PCMCIA PC Card ATA
device or as a 68-pin ATA device. This interface was
developed within the SFF committee and is now included in
ATA-2.
Basic Questions
---------------
### Where did ATA come from?
What we now call the ATA-1 interface was developed for Compaq
many years ago by Imprimus (then part of CDC, now part of
Seagate) and Western Digital. The first ATA-1 hard disk
drives were made by Imprimus but it was Conner that made the
interface so popular.
### How is ATA different from MFM?
From the host software standpoint, ATA is very much like the
Task File interface used by MFM controllers. A properly
written host software driver should not notice any difference
between the MFM Task File interface and the ATA Command Block
interface while doing basic commands such as Read/Write
Sectors.
At the hardware level, ATA uses a single cable between a host
bus adapter and the ATA device, where the MFM controller
interface uses two cables between the controller and the
drive.
In the MFM environment, the controller is one piece of
hardware and the drive another piece of hardware. Most likely
these two pieces of hardware are from different vendors. The
MFM controller is dependent on the design of both the host bus
and on the drive.
In the ATA environment, the host adapter is the one piece of
hardware that is dependent on the host system bus design. The
ATA interface is (mostly) system independent. All of the
hard disk controller and drive logic is contained in the ATA
device hardware. This gives the hard disk designer complete
control over both the controller and drive functions.
### Why is ATA so popular?
Two basic things make ATA so popular today: cost and hard
disk drive technology. An ATA-1 host adapter is cheap,
usually much less than $25US and it uses only one cable. On
the technology side, current hard disk features, such as,
defect handling, error recovery, zone recording, cache
management and power management require that the controller be
fully integrated with the read/write channel, the servo system
and spindle hardware of the disk drive.
### What are the basics of the ATA interface?
The ATA interface is a very simple interface based on an ISA
bus I/O device architecture. The interface consists of two
sets of I/O registers, mostly 8-bit, for passing command and
status information. The registers are like a set of mail
boxes with a door on front and back connected such that both
doors can not be open at the same time. The front door is
open when the Busy bit in the Status register is zero and the
host can read and write the registers. The back door is open
when the Busy bit in the Status register is one and the ATA
device can read or write the registers.
The physical interface contains just enough signals for a 16
bit data bus, five register address bits, and a few control
signals like read register, write register and reset.
ATA devices look like traditional hard disk
drives even though some are not really a hard disc with
rotating platters. User data is recorded in 512 byte sectors.
Each sector has a sector address. There are two ways to
express sector addresses: by cylinder/head/sector (CHS) or by
logical block address (LBA). CHS is standard, LBA is optional.
### What is EIDE or Fast ATA?
Both are marketing programs used to promote various ATA-2
features, mostly the faster data transfer rates defined by
ATA-2.
---
WD defines EIDE as:
* Support for drives larger than 528MB.
* Support for two connectors to allow up to four drives.
* Support for CD-ROM and tape peripherals.
* Support for 11.1/16.6 Mbytes/second, I/O Channel Ready PIO
data transfers.
* Support for 13.3/16.6 Mbytes/second, DMA data transfers.
---
???Seagate defines Fast ATA as:
* Support for PIO mode 3 (11.1 MB/sec) and DMA mode 1(13.3
MB/sec).
* Support for Multi-sector [Read/Write Multiple] transfers.
* Support for >528 MB.
* Support for Identify Drive Extensions & Set Transfer Mode
Extensions.
* Backward compatibility with ATA-1.
---
What does all of this mean to us?
Support for the ATA-2 high speed PIO and DMA data transfer
modes is both a hardware and software issue.
Support for more than one hard disc controller (or ATA host
adapter) requires the BIOS and/or the operating system to
support more than one Task File or Command/Control Block
register set on the host bus.
The 528MB problem is due to the original design of the x86
BIOS which limits cylinders to 1024 and sectors to 63. The
ATA interface allows up to 65,535 cylinders, 16 heads and 255
sectors -- that's about 136GB (137GB if is LBA is used).
Support for devices over 528MB requires the BIOS and/or
operating system to support some form of CHS translation.
Note that LBA alone does not solve this problem (in fact,
LBA may make things more complex).
Support for CD-ROM and tape will probably be done via the
ATAPI interface which uses a different command structure than
ATA. That makes ATAPI another host software issue.
### What does an ATA-1 host adapter do?
An ATA-1 host adapter is a simple piece of logic whose main
purpose is to reduce the system bus address lines from 12 (or
more) down to 5. It may also buffer some signals giving some
degree of electrical isolation between the host bus (usually
an ISA or EISA bus) and the ATA bus. In ATA-1, the ATA
interface is controlled directly by the host bus so that all
timings are controlled by the host bus timing.
### What does an ATA-2 host adapter do?
This answer is complex because it depends on how smart your
ATA-2 host adapter is. First, an ATA-2 host adapter supports
the ATA-2 higher speed data transfer rates. That requires
that the host adapter is attached to something other than an
ISA or EISA bus. Second, an ATA-2 host adapter may perform
32-bit wide transfers on the host bus. This requires FIFO
logic and data buffers in the host adapter. Third, an ATA-2
host adapter may use a different data transfer protocol on the
host side than is used on the ATA device side.
! ### Can I put an ATA-2 device on an ATA-1 host adapter?
! ### Can I put an ATA-1 device on an ATA-2 host adapter?
The answer to both questions is yes, as long as the electrical
timing specifications of the device are not violated. In
general it is impossible for an ATA-1 host adapter to violate
the specifications of an ATA-2 device. It is possible for an
ATA-2 host adapter to violate the timing specifications of an
ATA-1 device but this is not common. Hoever, host adapter
hardware design errors or software driver bugs can cause such
a problem. The result will be corrupted data read or written
to the ATA-1 device.
! ### I have an ATA-2 host adapter with an ATA-2 device. I want to
! ### add an ATA-1 device to this host adapter. Can I run the ATA-2
! ### device in its ATA-2 data transfer modes?
Sorry, *NO* you can *NOT* run the new drive in its faster data
transfer modes. Be very careful, most of the ATA-2 host
adapter vendors don't have anything in their setup
documentation or software to prevent this sort of thing.
When you run the new drive at a data transfer speed that is
faster than the older drive can support, you are violating the
electrical interface setup and hold times on the older drive.
There is no telling what the older drive will do about this,
but you are asking for data corruption and other nasty
problems on your older drive.
### How many disk controllers and/or ATA host adapters and/or
### SCSI host adapters can I put in my system?
From a hardware standpoint -- as many as you want as long as
there are no I/O port address, memory address or interrupt
request signal conflicts. From a software standpoint it is a
whole different story.
First the simple x86 system hard disk controller
configurations...
a) 1 ATA with 1 or 2 drives at I/O port addresses
1Fxh/3Fxh using interrupt request 14 (IRQ14).
b) 1 ATA with 1 drive at I/O port addresses 1Fxh/3Fxh
using interrupt request 14 (IRQ14) plus a SCSI with 1 drive.
c) 1 SCSI with 1 or 2 drives.
Other configurations are possible but are most likely not
supported in the system or SCSI host adapter BIOS. And if its
not supported at the BIOS level, it is unlikely to be
supported by an operating system, especially DOS. The primary
reason the above configurations are so restrictive is that the
original IBM x86 BIOS supported only one MFM controller with a
maximum of 2 drives. This restriction was then coded into
much x86 software including many early version of DOS. The
configurations above work because they don't break this old
rule.
Just remember this -- most systems will always boot from
the first drive on the first controller. In a) that is
ATA drive 0, in b) that is ATA drive 0, in c) that is
SCSI drive 0.
And now the more complex configurations...
Once you go beyond the three configurations above all bets are
off. Most likely you will need operating system device
drivers in order to access any drives beyond the first two.
And now your real problems start especially if you like to run
more than one operating system!
If you do run more than one OS, then you need equivalent
drivers for each system if you would like to access all the
drives. Plus it would be nice if all the drivers configured
the drives in the same manner and supported all the possible
partitioning schemes and partition sizes. It would be
especially nice if a driver would not destroy the data in a
partition just because it did not understand the file system
format in that partition.
One of the things EIDE promotes is BIOS support for up to four
ATA devices -- 2 ATA host adapters each with 1 or 2 drives.
The first would be at I/O port addresses 1Fxh/3Fxh using
interrupt request 14 (IRQ14) and the second at I/O port
addresses 17xh/37xh using interrupt request 15 (IRQ15).
Acceptance of this configuration has not been spreading like
wild fire through the BIOS world.
Lets look at a two complex configurations...
a) 1 ATA with 2 drives and 1 SCSI with 1 or more drives.
Nice configuration. The ATA drives would be supported by the
system BIOS and the SCSI drives may be, could be, should be,
supported by the SCSI host adapter BIOS but probably not. So
in order to use the 2 SCSI drives you probably have to disable
the BIOS on the SCSI card and then load a device driver in
CONFIG.SYS. And because the SCSI BIOS is disabled, you then
need a SCSI driver for that other OS you run.
b) 2 ATA with 1 or 2 drives on each.
Also a nice configuration. But because the system BIOS
probably only supports the first controller address, you'll
need a DOS device driver loaded in CONFIG.SYS in order to
access the drives on the second controller. You'll need that
driver even if there is only one drive on the first
controller. You also need a similar driver to support the
second controller in your other OS.
Note: I understand that OS/2 does support both MFM/ATA
controller addresses and does allow up to four drives -- I
have not confirmed this for myself.
! ### Are disk drives the only ATA devices?
No. Over the years there have been ATA tape drives, ATA
CD-ROMS and other strange devices. Most of these are expected
to be added to an existing ATA host adapter as the second
device (device 1) with an existing ATA disk drive (device 0).
In general these require software drivers to operate with your
OS.
Now, we have ATAPI CD-ROM and tape devices that can be placed
on an ATA host adapter. And soon we should see system
motherboard BIOS support for booting from an ATAPI CD-ROM
device. The general idea is that an ATAPI device can coexist
with an ATA device on the same cable.
! ### What can be done to improve ATA device performance?
A difficult question. But the first step is usually to reduce
the number of interrupts that the host sees during a read or
write command. ATA disk drives have three types of read/write
commands:
* Read Sectors / Write Sectors -- These commands are the old
traditional data transfer commands. These commands generate
one interrupt to the host for each sector transfered. These
are PIO data in and PIO data out commands which use the host
processor to transfer the data.
* Read Multiple / Write Multiple -- These commands where
defined in ATA-1 but were not used very much until recently.
These commands generate one interrupt to the host for each
block of sectors transfered. The number of sector per block
is generally 4, 8 or 16. However, when 1 sector per block is
used, these commands are the same as the Read/Write Sectors
commands. These are PIO data in and PIO data out commands
which use the host processor to transfer the data.
* Read DMA / Write DMA -- These commands where defined in
ATA-1 but were not used very much until recently. The main
reason for not using them was that x86 system DMA transfer
rates are much slower than PIO. However, these command do
generate a single interrupt at the completion of the command.
Now see the next question...
! ### What else can be done to improve ATA device performance?
! ### -or-
! ### What is PIO mode "x" ?
An even more difficult question. The second step is usually
to increase the rate at which the host transfers data.
(Ahh... I can see the funny look on your face from here. You
are saying to yourself: "the rate at which the host transfers
data? doesn't this guy have things backwards?" Read on...)
The rate at which data is transferred to or from an ATA device
is determined by only one thing: the PIO or DMA cycle time
the host uses. No, the drive does not have much to do with
this! The only requirement is that the host not exceed the
minimum PIO or DMA cycle times that the device supports. For
example, during a PIO read command when the device signals an
interrupt to the host this means that the device is waiting
for the host to read the next sector or block of sectors from
the drive. The host must execute a REP INSW instruction to do
transfer the data. The rate at which the host executes this
instruction determines the PIO cycle time. Technically, for a
read command, the cycle time is the time from the host
assertion of the I/O Read signal to the next time the host
asserts the I/O Read signal.
Be careful when looking at the table below -- the data rate is
the data transfer rate achieved while transfering the sector
or block or sectors. It is an "instantanous" data rate. The
overall data transfer rate for a command includes many time
consuming events such as the amount of time the host requires
to process an interrupt. Note that on many fast ATA drives
today, the time it takes the host to process an interrupt is
frequently greater than the time required to transfer the
sector of block of sectors for that interrupt! It is not
uncommon for the host overhead to reduce the data rate to 1/2
or 1/3 of the instantanous rate shown here.
The ATA PIO modes are defined as follows:
PIO min cycle data comment
mode time rate
0 ???ns ?MB the rate at which a system
running at 4.77MHZ could
execute the REP INSW.
1 ???ns ?MB the rate at which a system
running at 6MHz could
execute the REP INSW.
2 240ns 8MB the rate at which a system
running at 8MHz could
execute the REP INSW.
3 180ns 13MB requires an ATA-2
host adapter.
4 120ns 16MB requires an ATA-2
host adapter.
The complete description of the PIO (and DMA modes is much
more complex and will be cover in more detail later in this
FAQ.
### Do I need BIOS or OS drivers to support more than 528MB?
Warning: Read the previous question before reading this one.
Maybe, probably, yes. The answer to this *very* complex and
will be discussed in detail in Part 2. Here is the brief
answer...
A traditional x86 system BIOS supports only CHS mode
addressing with cylinders limited to 1024, heads limited to 16
and sectors limited to 63. This allows addressing of drives
up to 528MB. These limitations come from the INT 13
read/write calls that combine a 10-bit cylinder number with a
6-bit sector number into a 16-bit register.
Note that this is entirely a software problem: the ATA
interface supports up to 65,535 cylinders, 16 heads and 255
sectors.
While the head number usually requires only 4-bits, up to 6 or
8 bits are available in the INT 13 interface. This fact has
allowed the SCSI folks to support big drives by increasing the
number of heads above 16. The SCSI host adapter BIOS converts
this "fake" CHS address to a different CHS or an LBA when it
issues a read/write command to the drive. The following table
shows some approximate drives sizes and the "fake" CHS
parameters that you may see from a SCSI BIOS:
cyl head sector size
1024 16 63 512MB
1024 32 63 1GB
512 64 63 1GB
1024 64 63 2GB
1024 256 63 8GB
The last entry represents the largest possible drive that
a traditional INT 13 BIOS can support.
The system BIOS folks *must* start supporting drives over
528MB in their BIOS by implementing some type of CHS
translation. To date, few systems have such BIOS. And here
is the bad part: Microsoft says that the BIOS *must*
support it in order to use it in their OS. The algorithm is
simple (but warning: this is not the complete algorithm!):
INT 13 input action ATA interface
cyl number "multiply" by n modified cyl number
head number "divide" by n modified head number
sector number nothing sector number
The value of n must be selected such that the modified head
number is less than 16.
LBA addressing at the hard disk drive level or at the BIOS or
driver level is another solution. This solution will probably
not be popular for several more years. It requires that the
BIOS people implement a new INT 13 interface, called the
Microsoft/IBM Extensions and that the OS people start using
this new BIOS interface. Few system BIOS support this
alternative interface today. Without this new interface, LBA
support at the hard disk drive level is not required.
So most of us have older systems without much possibility of
getting a BIOS upgrade, so what do we do? Well we must obtain
one of the many driver products that are on the market that
live in one of the disk boot sectors and "take over" the
system BIOS INT 13 with an INT 13 that supports the
translation. The biggest problem with this is that the
replacement INT 13 BIOS must live someplace in memory. For
DOS based systems, it can usually live at the top of the 640K
of memory and DOS is made to think that that part of memory,
usually around 8K bytes, does not exist. But the protected
mode OS's don't like this and usually wipe out the driver when
they load their kernel. So if you plan to run multiple OS's
on your system, buyer beware!
Then there is the Windows problem: the standard FastDisk
driver in Windows does *not* support such translation schemes
and can not be enabled. So make sure the driver you
purchase also comes with a Windows FastDisk replacement.
Buyer beware!
### Do I need BIOS or OS drivers to support the ATA-2 data
### transfer rates?
Warning: Read the previous two questions before reading this
one.
Maybe, probably, yes. The answer to this *very* complex and
will be discussed in detail in Part 2. Here is the brief
answer...
If you have a new ATA drive that supports the advanced PIO or
DMA data transfer rates (ATA-2 PIO Mode 3 or 4, or, ATA-2 DMA
Mode 1 or 2) then you also must have a new ATA host adapter
that attaches to the VL-Bus or PCI bus or some other high
speed bus (probably a 32-bit bus) in your system. That host
adapter has I/O registers of its own that are used to control
its advanced features. Controlling those advanced features
requires software -- either in the system INT 13 BIOS or in a
INT 13 BIOS on the host adapter card or in a driver loaded
via the boot record or later by your OS.
Depending on how that host adapter works you may also
need a Windows FastDisk replacement in order to use the high
speed data transfer modes in Windows.
Buyer beware!
### I just purchased a new high speed host adapter for my VL-Bus
### (or PCI bus) system and a new 540MB hard disk. How do I get
### full use out of all this new hardware?
Did you read the previous three questions?
You need BIOS or driver software and a Windows FastDisk
replacement. These *must* support both CHS translation
(because your drive is over 528MB) and the host adapter
hardware (to use the high speed data transfer rates).
Some drivers on the market today use LBA addressing on the
ATA interface to get over 528MB. This may make your disk
partition(s) unreadable by another OS.
Check the hardware and software specifications of the product
before you buy it! Ask lots of questions -- you probably get
lots of incorrect or misleading answers -- be prepared for
that! If you plan to run something other than DOS and
Windows, especially if you plan a "dual boot" or "boot
manager" environment, be real careful.
Buyer beware!
OPINION: I know of only one product that supports all of this
new hardware, supports over 528MB *and* supports most of the
current OS's that are shipping including several in shipping
in beta form. The product is from a small two person company
that is trying to sell the product on an OEM basis and not in
the retail market. - Hale Landis <hlandis@ibm.net>
/end part 1/
NOTICE (18 Dec 99): See the file README.TXT!
How it Works -- Partition Tables
Part 2 of 2
Version 1j -- 18 Dec 99
by Hale Landis <hlandis@ibm.net>
PARTITION TYPE CODES
The following table of partition type codes was compiled from
many sources including information from kind people at IBM and
Microsoft.
Both IBM and Microsoft keep the partition type code lists and
both have until recently assigned type codes. However, there
apparently is no formal agreement between these two companies and
their lists are not always in sync or up to date. It should be
noted that the lists I obtained from IBM and Microsoft are fairly
short and show most of type type codes as "available" or
"reserved". My guess is that many type codes have been used
without the knowledge of the IBM or Microsoft. This is probably
the case since neither IBM or Microsoft have published a phone
number or email address to contact if you wanted to request a
type code. Chaos is the keyword here.
It now appears that neither company is assigning new type codes.
I have also been told that it is now recommended that anyone
defining a new partition type, or more correctly stated, anyone
defining a new file system type, should use partition type 07 and
use the first block(s) of the partition to fully define the file
system type. Of course, the appearance of a new type code could
cause problems for older versions of FDISK, various older
operating system device drivers and disk utility programs. (This
new use of type code 07 doesn't explain the recent "assignment"
of type codes 0E and 0F.)
[If you know of a type code assignment that is not listed here
please let me know about it ASAP. Thanks, Hale]
Note that several type codes have multiple uses (for example, see
code 08). Also note that there is some question about the use of
some codes as denoted by a '?' in the description.
Code Description
00 Unused partition table entry
01 DOS, 12-bit FAT
02 XENIX root
03 XENIX user
04 DOS, 16-bit FAT
05 Extended partition (includes other partition types)
06 DOS, >32MB support, up to 64K Allocation unit
07 See partition boot record(s) for file system type:
could be QNX, OS/2 HPFS, Windows NT NTFS, Unix, ...
08 OS/2 (thru Version 1.3 only)
08 DELL partition spanning multiple drives (array)
08 Commodore DOS
08 AIX boot? or file system?
09 AIX boot? or file system?
09 Coherent swap
0A OS/2 Boot Manager
0A OPUS
0A Coherent swap
0B FAT32
0C FAT32 LBA (SEE NOTE BELOW)
0D ? (perhaps a type 07 LBA, SEE NOTE BELOW)
0E FAT16 LBA (SEE NOTE BELOW)
0F Extended partition LBA (SEE NOTE BELOW)
NOTE: Partitions types 0C, 0E and 0F (perhaps 0D too)
REQUIRE that the system's INT 13 BIOS support the
IBM/Microsoft/Phoenix extended/enhanced functions calls
(AH=4x). In these partition table entries the CHS fields are
NOT used and are generally set to maximum values (all 1 bits)
in each CHS field. What this means is that some day there
will be an LBA type partition for all the other partition
types that are listed here and still in use by some system.
This will certainly use up many of the currently unused type
codes!
10 OPUS
11 OS/2 Boot Manager: Inactive type 01
12 Compaq diagnostics
13 Available for assignment
14 OS/2 Boot Manager: Inactive type 04
14 Novell DOS 7.0 FDISK (result of bug in FDISK?)
15 Available for assignment
16 OS/2 Boot Manager: Inactive type 06
17 OS/2 Boot Manager: Inactive type 07
18 AST Windows swap file
18 Compaq System Diagnostics
19 - 1F Available for assignment
20 Available for assignment
21 Reserved
22 Available for assignment
23 Reserved
24 NEC version of MS-DOS
25 Available for assignment
26 Reserved
27 - 2F Available for assignment
30 Available for assignment
31 Reserved
32 Available for assignment
33 Reserved
34 Reserved
35 Available for assignment
36 Reserved
37 - 3B Available for assignment
3C PowerQuest PartitionMagic recovery partition
3D - 3F Available for assignment
40 VENIX :Venix 80286
41 Personal RISC Boot
41 PowerPC boot
42 Secure File System (Peter Gutmann)
43 - 4E Available for assignment
4F Oberon boot and data
50 OnTrack Disk Manager (read-only)
51 OnTrack Disk Manager (write-only)
51 Novell
51 OnTrack Disk Manager (read-only)
52 CP/M
52 Microport
53 OnTrack Disk Manager (write-only)
54 OnTrack Disk Manager (DDO)
55 Available for assignment
56 GoldenBow VFeature
57 - 5F Available for assignment
60 Available for assignment
61 SpeedStor
62 Available for assignment
63 UNIX System V/386
63 Mach, MtXinu BSD 4.3 on Mach
63 GNU HURD
64 Speedstore
64 Novell
65 Novell 286 Netware
66 Novell 386 Netware
67 Novell
68 Novell
69 Novell
6A - 6F Available for assignment
70 DiskSecure Multi-Boot
71 Reserved
72 Available for assignment
73 - 74 Reserved
75 PC/IX
76 Reserved
77 - 79 Available for assignment
7A - 7F ? (probably "available for assignment", these codes
are not shown in the IBM or Microsoft lists!)
80 Minix (ver. 1.4a and earlier)
81 Minix (ver. 1.4b and later)
81 Mitac Advanced Disk Manager
81 Linux
82 Prime
82 Linux swap
82 Solaris UNIX
83 Linux ext2fs
84 OS/2 hiding a type 04
85 Available for assignment
86 Reserved
87 HPFS FT mirrored partition
88 - 8F Available for assignment
90 - 92 Available for assignment
93 Ameba file system
94 Ameba bad block table
95 - 98 Available for assignment
99 Mylex EISA SCSI
9A - 9F Available for assignment
A0 Available for assignment
A1 Reserved
A2 Available for assignment
A3 - A4 Reserved
A5 FreeBSD
A6 Reserved
A7 - A8 Available for assignment
A9 NetBSD
AA - AF Available for assignment
B0 Available for assignment
B1 Reserved
B2 Available for assignment
B3 - B4 Reserved
B5 Available for assignment
B6 Reserved
B7 BSDI file system or secondarily swap
B8 BSDI swap or secondarily file system
B9 - BD Available for assignment
BE Solaris boot partition
BF Available for assignment
C0 Available for assignment
C1 DR-DOS LOGIN.EXE-secured 12-bit FAT
C2 BlueSky Innovations boot manager
C3 BlueSky Innovations boot manager
C4 DR-DOS LOGIN.EXE-secured 16-bit FAT
C5 Available for assignment
C6 DR-DOS LOGIN.EXE-secured Huge
C7 HPFS FT disabled mirrored partition
C7 Cyrnix Boot
C8 - CF Available for assignment
D0 - D7 Available for assignment
D8 CP/M 86
D9 - DA Available for assignment
DB Concurrent DOS, CP/M and CTOS
DC - DF Available for assignment
E0 Available for assignment
E1 Speedstore
E2 Available for assignment
E3 Storage Dimensions
E4 Speedstore
E5 - E6 Reserved
E7 - EA Available for assignment
EB BeOS BFS
EC - EF Available for assignment
F0 Available for assignment
F1 Storage Dimensions
F2 DOS 3.3+ second partition
F3 Reserved
F4 Speedstore
F4 Storage Dimensions
F5 Available for assignment
F6 Reserved
F7 - FD Available for assignment
FE IBM PS/2 IML, LANstep
FF Xenix(?) Bad Block Tables
/end part 2 of 2/
Date: 18 December 1999
------------------------------
README.TXT FILE FOR ALLHIW.ZIP
------------------------------
This is the README.TXT file for Hale's ALLHIW.ZIP file.
You will find the latest ATA/ATAPI information at Hale's web site
http://www.ata-atapi.com
including the latest ATA/ATAPI information and the latest HIW
documents and the latest version of the ALLHIW.ZIP file.
The best way to use these files is to point your browser at the
file HIW.HTM.
----------------------
CONTENTS OF ALLHIW.ZIP
----------------------
ALLHIW.ZIP contains the HIW*.HTM files from Hale's web site. You
should be able to point your browser at the file HIW.HTM and view
the HTM file just as if you were online. These files are the
most up-to-date versions of Hale's HIW documents.
ALLHIW.ZIP also contains the following *.TXT files. These are
the older versions of Hale's HIW documents. These are mostly
up-to-date but they are slowly being replaced by the HIW*.HTM
files.
ATAFAQ1.TXT Hale's ATA FAQ,
see HIWFAQ.HTM.
CHSTRAN1.TXT CHS translation part 1,
see HIWCHS.HTM.
CHSTRAN2.TXT CHS translation part 2,
see HIWCHS.HTM.
DOSBOOT.TXT Disassembly of a DOS boot sector,
see HIWDOS.HTM.
MBR.TXT Disassembly of a Master Boot Record,
see HIWMBR.HTM.
OS2BOOT.TXT Disassembly of an OS/2 Boot Record,
see HIWOS2.HTM.
PARTTAB1.TXT Partition Tables part 1,
see HIWTAB.HTM.
PARTTAB2.TXT Partition Tables part 2,
see HIWTAB.HTM.
README.TXT this file.
------------------------------------
EMAIL ADDRESSES AT SUGS.TALISMAN.COM
REPLACED BY ATA-ATAPI.COM WEB SITE
------------------------------------
ATA-ATAPI.COM is now online:
http://www.ata-atapi.com
This web site replaces all of the email addresses listed below.
atact@sugs.talisman.com
atademo@sugs.talisman.com
atadoc@sugs.talisman.com
atadrvr@sugs.talisman.com
hiw@sugs.talisman.com
info@sugs.talisman.com
=====
I hope you find the information you need in these files!
Hale Landis
hlandis@ata-atapi.com
/end/
<html>
<head>
<title>ATA-ATAPI.COM -- Hale's HIW: How It Works Documents</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">ATA-ATAPI.COM
<BR>Hale's HIW: How It Works Documents</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<H2><FONT COLOR="blue">The HIW or How It Works Documents</FONT></H2>
<p>Over the years Hale has assembled a set of How It Works
documents, the HIW documents, that describe various subjects
including how disk drives work and how they are supported on
x86 computer systems. The documents are available as a
collection of plain ASCII text documents in the file
ALLHIW.ZIP and they are available online in HTML.</p>
<p>NOTE: THERE ARE DIFFERENCES BETWEEN THE DOCUMENTS IN
ALLHIW.ZIP AND THE ONLINE HTML DOCUMENTS. THIS WILL BE
CORRECTED SOON.</p>
<p><A HREF="allhiw.zip" NAME="T1"><FONT COLOR="green">
Get the ALLHIW.ZIP file.</FONT></A></p>
<p>The online HTML versions are:</p>
<ul>
<li><a href="hiwfnf.htm"><FONT COLOR="green">
Facts and Fiction</FONT></a></li>
<li><a href="hiwfaq.htm"><FONT COLOR="green">
Hale's ATA FAQ</FONT></a></li>
<li><a href="hiwchs.htm"><FONT COLOR="green">
CHS Translation</FONT></a> </li>
<li><a href="hiwtab.htm"><FONT COLOR="green">
Partition Tables</FONT></a></li>
<li><a href="hiwmbr.htm"><FONT COLOR="green">
Master Boot Record</FONT></a></li>
<li><a href="hiwdos.htm"><FONT COLOR="green">
DOS Floppy Disk Boot Sector</FONT></a></li>
<li><a href="hiwos2.htm"><FONT COLOR="green">
OS2 Boot Sector</FONT></a></li>
</ul>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: Facts and Fiction</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>Facts and Fiction</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">Facts and Fiction</font></h2>
<ul>
<li><a href="#T2"><FONT COLOR="green">
Who Is Hale Landis?</font></a></li>
<li><a href="#T4"><FONT COLOR="green">
What is a Mega Anyway?</font></a></li>
<li><a href="#T6"><FONT COLOR="green">
Standard(?) I/O Port Addresses and Interrupt Numbers</font></a></li>
<li><a href="#T8"><FONT COLOR="green">
Logical Block Addressing (LBA)</font></a></li>
<li><a href="#T10"><FONT COLOR="green">
Which is better: LARGE or LBA?</font></a></li>
<li><a href="#T12"><FONT COLOR="green">
Even More About LBA</font></a></li>
<li><a href="#T14"><FONT COLOR="green">
Low Level Formating</font></a></li>
<li><a href="#T16"><FONT COLOR="green">
Zone Bit Recording (ZBR)</font></a></li>
<li><a href="#T18"><FONT COLOR="green">
Dual Channel ATA Host Adapters and Data Corruption</font></a></li>
<li><a href="#T20"><FONT COLOR="green">
Why Disk Drives Break</font></a></li>
<li><a href="#T22"><FONT COLOR="green">
Plug and Play (PnP)</font></a></li>
<li><a href="#T24"><FONT COLOR="green">
Thermal Calibration and AV Drives</font></a></li>
</ul>
<h3><a name="T2"><font color="green">Who Is Hale Landis?</font></a></h3>
<p>(Ahhh... A very good question...)</p>
<p>Hale Landis is an ATA/ATAPI interface consultant. Hale is
available to teach classes about these interfaces or help
design hardware or software for these interfaces. Hale has
25+ years experience creating diagnostic test software for
disk storage subsystems and devices. He is very active in the
ATA and ATAPI standards committee efforts and is a member of
the ANSI NCITS T13 committee.</p>
<p>Over the years Hale has been very active in getting the
ATA/ATAPI standards updated and has worked very hard to keep
this interface simple, fast and cheap. The world doesn't need
another SCSI or 1394!</p>
<h3><a name="T4"><font color="green">What is a Mega anyway?</font></a></h3>
<p>And here is something that Hale really hates...</p>
<p>Someone once wrote:</p>
<p>HD manufacturers think 1MB = 1e6 bytes, not 1048576
bytes.</p>
<p>And Hale said:</p>
<p>The HD manufacturers are right: 1MB is 1000000
bytes—it IS NOT 1048576 bytes! Mega means 1000000 (just
like kilo means 1000).</p>
<p>Is a 1MHz a frequency of 1048576 cycles/second? NO! It is
1000000 cycles/second. Is a kilometer 1024 meters? NO! It
is 1000 meters.</p>
<p>Mega, Kilo, etc are ISO defined terms that predate the
computer industry by many decades!</p>
<h3><a name="T6"><font color="green">Standard(?) I/O Port Addresses and Interrupt Numbers</font></a></h3>
<p>The following table shows the most commonly supported I/O
port addresses and IRQ numbers used for PC ATA (IDE/EIDE) host
adapters.</p>
<table width="60%" border="1" cellspacing="1" cellpadding="5" align="center">
<tr>
<td width="25%" align="center"><b>Interface Number</b></td>
<td width="25%" align="center"><b>CS0- Decode</b></td>
<td width="25%" align="center"><b>CS1- Decode</b></td>
<td width="25%" align="center"><b>IRQ number</b></td>
</tr>
<tr>
<td width="25%" align="center">1</td>
<td width="25%" align="center">01F0h-01F7h</td>
<td width="25%" align="center">03F6h-03F7h</td>
<td width="25%" align="center">14</td>
</tr>
<tr>
<td width="25%" align="center">2</td>
<td width="25%" align="center">0170h-0177h</td>
<td width="25%" align="center">0376h-0377h</td>
<td width="25%" align="center">15</td>
</tr>
<tr>
<td width="25%" align="center">3</td>
<td width="25%" align="center">01E8h-01EFh</td>
<td width="25%" align="center">03EEh-03EFh</td>
<td width="25%" align="center">12 or 11</td>
</tr>
<tr>
<td width="25%" align="center">4</td>
<td width="25%" align="center">0168h-016Fh</td>
<td width="25%" align="center">036Eh-036Fh</td>
<td width="25%" align="center">10 or 9</td>
</tr>
</table>
<p>Now some history and notes...</p>
<p><b>Interface number 1 -- The Primary</b><br> <br> The
addresses and IRQ number for the first interface have been
well established since the first IBM PC/AT system in 1984.
This is the address and IRQ used by the MFM controller in the
original IBM PC/AT. All PC software (BIOS and OS) support the
primary host adapter (because they support an MFM, RLL or ESDI
controller using this configuration). Remember that ATA
(IDE/EIDE) is designed to operated just like an old MFM
controller (of course there are new features in ATA that MFM,
RLL and ESDI did not support).</p>
<p><b>Interface number 2 -- The Secondary</b><br> <br> During
1995 support for second interface has become very well
established. Some implementations of this interface number
have used IRQ’s other than 15 in the past. Today, IRQ 15
has become the accepted standard for the secondary
interface.</p>
<p><b>The other interfaces</b><br> <br> The addresses and
IRQ’s used by the 3<sup>rd</sup> and 4<sup>th</sup>
interfaces is not very standard. Usually it is the ATA (IDE)
interface on some other type of card, such as a sound card,
that is used for the 3<sup>rd</sup> or 4<sup>th</sup> ATA host
adapter in a system. There is no standardized BIOS or OS
support for such configurations and usually an OS device
driver is required to access these ATA host adapter
addresses.</p>
<h3><a name="T8"><font color="green">Logical Block Addressing (LBA)</font></a></h3>
<p><b>LBA DOES NOT SOLVE THE >528MB PROBLEM!</b></p>
<p>There continues to be a large amount of very false
information floating around in newsgroups concerning LBA
addressing on ATA (IDE) drives.</p>
<p>Please, lets get this correct...</p>
<ul>
<li>LBA has NOTHING to do with fixing the >528MB problem. </li>
<li>LBA does NOT solve the >528MB problem. </li>
<li>It is not true that LBA is needed to support a drive >528MB. </li>
</ul>
<p>An INT 13H BIOS that does CHS translation is the ONLY way
to support a drive >528MB today. CHS translation enables
the use of >16 heads with <=1024 cylinders at the INT
13H interface while using <=16 heads and >1024 cylinders
(or maybe LBA) at the drive interface.</p>
<p>FDISK uses CHS. It does not use LBA. It does not know (or
care) if LBA is being used at the device interface. FDISK can
not address any part of a disk that can not be accessed in CHS
mode.</p>
<p>Microsoft operating systems WILL NOT use any part of a disk
that can not be accessed in CHS mode. Read the Microsoft
documentation carefully. OS/2 and Linux do have a special
FDISK "hack" that allows partitions to span or be
beyond the 528MB boundary even on systems that don’t have
a CHS translating BIOS and even on drives that don’t
support LBA. Be very careful when using this feature of OS/2
or Linux.</p>
<p>LBA provides only a slight performance improvement for some
protected mode operating systems, HOWEVER, those systems MUST
still boot in CHS mode and use CHS mode to understand the
drive configuration data returned by INT 13H AH=08H and to
understand the partition layout of the drive. If an OS
decides to use LBA addressing it MUST not attempt to access
any sectors that are beyond those that can be accessed by the
CHS mode returned by INT 13H AH=08H. Therefore, the size of a
drive in LBA mode is the same as the size in CHS mode. If the
INT 13H BIOS does not support drives >528MB then LBA mode
can not be used make the drive look >528MB either.</p>
<p>So, once again, LBA does not solve the >528MB problem.
Don’t be confused by the the BIOS vendors that claim LBA
solves the >528MB problem. They are confusing you by using
LBA to explain that their BIOS is really doing CHS
translation. It is very unfortunate that they are using LBA
so incorrectly. We can probably thank Western Digital’s
EIDE Implementation Guide for starting this confusion.</p>
<p>NOTE: If you are about to send me email to tell me how
wrong I am, please see my "How It Works" series of
documents that describe how the MBR and boot sectors work, or,
disassemble the master boot record on your system and look at
how your system boots. You will find that it does not use LBA
at all. If after doing this you still think I’m wrong,
then send me email. Thanks!</p>
<h3><a name="T10"><font color="green">Which is better: LARGE or LBA?</font></a></h3>
<p>Hmmm... The truth is this: The LBA option is not stable
and is not the "standard" that will survive in the
future. I’ve posted several articles in the past about
this but the misleading information comes faster than I and
others can keep up.</p>
<p>The truth is that the IBM/MS/Phoenix extended/enhanced BIOS
specification <b>is</b> the future "standard" and it
is being widely implemented. This "standard" does
not require the use of LBA at the drive interface. Using LBA
at the device interface never did and never will solve the
>528MB (>1024 cylinder) problem. Only CHS translation
at the INT 13 interface or LBA at the INT 13 interface solves
this problem.</p>
<p>The truth is that the LBA BIOS option is a poorly designed
and very poorly documented Western Digital idea that should
have never been adopted by the PC industry. Even WD
doesn’t actively support it any more.</p>
<p>Maybe this table will help everyone understand this
problem... </p>
<table width="90%" border="1" cellspacing="1" cellpadding="5" align="center">
<tr>
<td width="30%"><b>drive size</b></td>
<td width="30%"><b>INT 13 interface </b></td>
<td width="40%"><b>IDE/EIDE drive interface</b></td>
</tr>
<tr>
<td width="30%"><528MB and <1024 cyl</td>
<td width="30%">any old BIOS works -- no CHS translation is required.</td>
<td width="40%">CHS used here is the same as the CHS used at the INT 13 interface.</td>
</tr>
<tr>
<td width="30%">>528MB or >1024 cyl but less than 8GB</td>
<td width="30%">CHS translation required -- keep cylinders under 1024 and use more than 16 heads
<p>implementation #1 is described by IBM/MS/Phoenix documents.</p>
<p>implementation #2 is described by the WD EIDE guide.</p></td>
<td width="40%">two possible implementations exist:
<p>#1 the "standard" or LARGE
implementation uses CHS at the drive interface and
generally gives better performance. </p>
<p>#2 the WD EIDE BIOS or LBA
implementation uses LBA at the drive interface (for no
good reason). This is not the "standard" of the
future and it can confuse some OS device drivers -- this
is why so many versions of the Windows FastDisk (32-bit
disk access) driver exist today. This BIOS implementation
does not work for drives of 8GB or bigger.</p></td>
</tr>
<tr>
<td width="30%">8GB or bigger</td>
<td width="30%">IBM/MS/Phoenix
"standard" is the only thing that works. LBA is
used at the INT 13 interface!</td>
<td width="40%">either
CHS or LBA can be used at the device interface.</td>
</tr>
</table>
<p><b>Note:</b> The IBM/MS/Phoenix "standard" works
for all drive sizes including drives with up to 2^64 sectors
(really big drives!). The WD LBA BIOS implemetation fails
when drives get to 8GB and it also doesn’t include any of
the Plug-and-Play stuff that future operating systems will
require.</p>
<p>I really think that someone at WD got confused about how
LBA should be used. It appears to me that this WD person (or
persons) thought that using LBA at the device interface would
solve the big disk problem. What they overlooked was this:
LBA at the INT 13 interface is what really solves the big disk
problems. LBA at the device interface isn’t needed to
solve any big disk problem (even for 100GB drives)!</p>
<p>As I’ve said before, those of you that are using the
LBA option in your BIOS setup <b>may</b> find that some day
you’ll have to back up all your data and switch to the
LARGE option in order to remain compatible with the real BIOS
"standard".</p>
<h3><a name="T12"><font color="green">Even More About LBA</font></a></h3>
<p><b>What does LBA do for me?</b><p>
<p>Some people in this message thread seem to be confused here
about the use of LBA to:</p>
<ol>
<li>get around the 528MB problem,</li>
<li>reformatting the disk because of LBA,</li>
<li>performance increase due to LBA.</li>
</ol>
<p>I’m not sure were all this started but, lets take a
brief look at each of these... </p>
<ol>
<li>LBA does not solve the >528MB problem. Only a BIOS
with an INT 13 that does CHS translation solves this
problem. How the BIOS sends sector addresses to the disk
drive, as CHS or as LBA, should not change the CHS that
is used at the INT 13 interface.<br>
<br>
Yes, there is at least one BIOS type out there that has a
problem with this: any BIOS based on the Western Digital
EIDE Implementation Guide may change the CHS used at the
INT 13 interface when the use of LBA at the device
interface is turned on or off. This is a very flawed
implementation and should be avoided OR if you have such
a BIOS, NOT do change the LBA setting in the CMOS setup
once you have partitioned your disk and installed your
software.</li>
<li>As long as the CHS at the INT 13 interface does not
change, it should make not difference if CHS or LBA is
being used at the drive’s interface. CHS and LBA are
completely interchangeable at the drive interface on a
command by command basis.<br>
<br>
However, beware of these WD EIDE BIOS types. These BIOS
incorrectly change the CHS used at the INT 13 interface
when LBA use at the drive interface is changed. This
should not be. This is the major flaw in the WD EIDE BIOS
type. It represents a serious threat to your data!</li>
<li>LBA use will not give any great performance increase. In
fact, on some drives it may decrease performance. The
disk drive industry has had many years to optimize the
use of CHS at the drive’s interface. Many drives do
NOT convert the CHS at the interface to an LBA
internally. In these drives, using LBA at the
drive’s interface just causes the drive more
overhead as it must convert the LBA to a CHS before it
can proceed with the command.</li>
</ol>
<p><b>Is LBA slower?</b><p>
<p>Someone once posted this on UseNet:</p>
<p>>..., no it is not slower, just a different geometry
translation, >and at the same speed as long as the system
supports LBA, >and is set up correctly.</p>
<p>Beware... LBA <b>can</b> be slower for two reasons:</p>
<ol>
<li>Conversion of INT 13 CHS input to an LBA at the device
interface (when LBA is used in BIOS setup) usually
requires more instructions than conversion of INT 13 CHS
input to another CHS at the device interface (when CHS
translation is used in BIOS setup, aka LARGE).</li>
<li>LBA processing in many drives is slower than CHS
processing (this will change as drive vendors optimize
their hardware and firmware for LBA in future drives).
Many people can measure a slight difference in
performance between CHS and LBA today. </li>
</ol>
<h3><a name="T14"><font color="green">Low Level Formating</font></a></h3>
<p><b>What does low-level format do on an ATA
(IDE) drive?</b></p>
<p><b>Answer:</b> Depends on how old the ATA drive is and who
made it.</p>
<p>Lets talk about what low-level formatting is...</p>
<p>In the old days of MFM, RLL and ESDI, a new hard disk drive
was just like a new unformatted floppy—there was nothing
on it. The drive had to be connected to a controller and the
controller had to be told how many sectors per track to lay
down on each track—this is the same thing you do when you
format a floppy these days. Formatting of each track creates
the inter sector gaps, the sector ID fields and the sector
data fields. Most MFM/RLL/ESDI controllers include a small
low-level formatting program in ROM. You generally use DOS
DEBUG to enter this program.</p>
<p>Lets talk about what high-level formatting is...</p>
<p>High level formatting creates a file system within a
partition of a hard disk or on a floppy. This process writes
the initial version of the boot record, root directory and
file allocation table (FAT).</p>
<p>Here is a bit of confusion—when you use the DOS FORMAT
command to format a floppy, you are doing BOTH a low-level and
high level format at the same time. When you use the DOS
FDISK and FORMAT commands to format a hard disk, all you are
doing is the high level format.</p>
<p>Back to hard disks...</p>
<p>MFM/RLL/ESDI and some older ATA drives use the same
controller command code, known as Format Track, to do the
low-level format of a track. This command is issued once for
each track on the hard disk. This command writes the inter
sector gaps, the sector ID fields and the sector data fields.
Each sector on a track has an ID—an 8-bit binary number
usually starting at 1. The order is which the sector IDs are
written determines the interleave. If the sector IDs are
written as 1, 2, 3, ..., n, this is 1-to-1 interleave. When
written as 1, n, 2, n+1, 3, ..., n-1, you have 2-to-1
interleave. MFM controllers usually used 2-to-1 or 3-to-1
interleave with 17 sectors per track. RLL usually used 26
sectors per track.</p>
<p>MFM/RLL/ESDI also would allow "marking" a sector
"bad" as the sector ID field was being created.
This would cause a very special kind of error on any read or
write command that attempted to access this sector—a Bad
Block error. When you run the DOS FORMAT program, it reads
every sector in the partition looking for "bad"
sectors and sectors with uncorrectable data errors. Such
sectors then cause clusters of sectors in the FAT to be marked
bad and DOS will never use them. Other systems, such as OS/2
HPFS, Unix, etc, keep bad block lists as part of their file
system data and they also do not access these "bad"
sectors.</p>
<p>MFM/RLL/ESDI drive technology was generally based on using
a stepper motor to position the read/write heads. Over time
the bearings in the drive would wear and read/write errors
would appear because the stepper motor was no longer
positioning the read/write heads in the right place. The
solution was to do a low-level format again.</p>
<p>New drives don’t use stepper motors and instead use
embedded servo bursts in between the sectors. These burst are
used to locate tracks and sectors and to keep the read/write
heads properly positioned even when the drive’s bearing
are worn. More important is keeping the read/write heads
properly positioned under a wide range of temperatures.
Thermal expansion can change the length of the arm the
read/write head is attached to by as much as 5 or 10 tracks!
This is the reason you hear new drives doing a lot of thermal
calibration (that strange seeking noise you hear when there
should be no disk activity).</p>
<p>The servo bursts are written at the factory in a very
controlled environment using some very expensive equipment.
The drive alone can not recreate these servo data bursts.
Likewise, because most drives are now zone recorded (they have
different number of sectors per track at different locations
on the media), the inter sector gaps, sector ID fields and
sector data fields are also written at the factory and can not
be recreated later. Some drives may soon do away with the
sector ID fields and some of the gaps in order to increase
data storage capacity.</p>
<p>Now for the history the ATA Format Track command...</p>
<p>Early ATA drives did not really implement the Format Track
command—it was thought to be obsolete (and it was). What
was implemented was a simple writing of some data pattern into
each sector of each track formatted. Most drives did not
support marking sectors bad. However, the disk drive industry
is driven by features and slowly, one-by-one, the hard disk
vendors started implementing the command such that it did the
same thing as in MFM/RLL/ESDI. Then someone implemented the
ability to "reassign" a sector’s address to a
different physical sector on the disk. Never mind that there
were <b>no</b> programs then and few now that use the Format
Track command to do this.</p>
<p>The original ATA specification (now at rev 4.0c, also known
as ATA-1) documents the "full" function Format Track
command but leaves it to the drive vendor to decide what a
drive will really do. It recommends a minimum action of
writing binary zero into the data field of each sector
formatted. The ATA-2 specification says that the function of
the command is "vendor specific"—it
doesn’t even recommend the minimum action of writing
binary zero data—a major step towards (finally) making
the command obsolete. </p>
<p><b>So what does low-level formatting do on a modern ATA drive?</b></p>
<p>Assuming that you can find a program that really does issue
the ATA Format Track command your ATA drive probably
doesn’t do anything other than write some data pattern,
maybe binary zeroes, into the data field of every sector on
the drive. That is no different than just using the normal
write command and writing data into every sector on the drive.
In general, low-level formatting of an ATA drive is just a big
waste of time.</p>
<p>So several things have gotten us to this time and
place:</p>
<ol>
<li>No stepper motors.</li>
<li>Very close track spacing.</li>
<li>Embedded servo data.</li>
<li>Zone recording.</li>
<li>Few programs that use the Format Track command.</li>
<li>The Format Track command DOUBLES the amount of
firmware in a drive.</li>
<li>And then there is the failure rate problem... There
are two major components in a disk drive—the
Head/Disk Assembly (HDA, contains the media and
read/write heads) and the printed circuit board with
the electronics—which fails most frequently?</li>
</ol>
<p><b>THE ELECTRONICS!</b></p>
<p><b>Wait a minute...</b> Are you trying to say the
electonics are more unreliable than the HDA (media and
heads)?</p>
<p>In general the failure rate for the electronics is slightly
higher than the failure rate of the media and heads. High
temperatures and heating/cooling cycles cause failure of the
little gold wires inside the chips. This could be a good
reason to make sure your drive has adequate cooling and leave
it on most of the time. But heat is the big problem
here—it is just a matter of time before the HDA or the
electronic will fail. Of course rough handling (like dropping
the drive) will probably not damage the electronics but will
damage heads or media.</p>
<h3><a name="T16"><font color="green">Zone Bit Recording (ZBR)</font></a></h3>
<p>ZBR maintains a constant data bit density across the disk
surface. This done by placing more sectors on tracks at the
outside of the disk and fewer sectors at the inside edge of
the disk. Typically, drives have >60 sectors on the
outside tracks and <40 on the inside tracks. This does
cause the disk’s raw data rate to change. The data rate
is higher on the outside than on the inside. Most ZBR drives
have at least three zones. Some my have 30 or more. All of
the tracks within a zone have the same number of sectors per
track.</p>
<p>ZBR and embedded servo data are the two major reasons you
can’t low level format drives anymore.</p>
<h3><a name="T18"><font color="green">Dual Channel ATA Host Adapters and Data Corruption</font></a></h3>
<p><b>WARNING! POSSIBLE SYSTEM HANGS AND DATA CORRUPTION
PROBLEMS!</b></p>
<p>If you have a dual channel ATA (IDE) host adapter (or you
are thinking of buying one) either an add in card or on a new
motherboard, read this.</p>
<p>First, ATA is the real name for IDE or EIDE.</p>
<p>Second, dual channel ATA host adapters are two ATA host
adapters in one package. These are add in cards with two ATA
connectors or a motherboard with two ATA connectors. You can
attach up to two ATA devices per host adapter (up to two
devices per cable).</p>
<p>Normally one host adapter will be assigned to the
"primary" I/O addresses (1F0-1F7H and 3F6H) and the
other host adapter will be assigned to the
"secondary" I/O addresses (1790177H and 376H).</p>
<p>A single ATA host adapter looks like this internally:</p>
<pre><font face="Courier New">
ATA interface
|
host |
bus V
|| +---------+ control +----------+ +----------+
|| | | signals | | | |
|| | |<------------->| |<-->| |
|| | host | | drive | | drive |
||<=====>| adapter | 16-bit | 0 | | 1 |
|| | logic | data bus | (master) | | (slave) |
|| | |<------------->| |<-->| |
|| | | | | | |
|| +---------+ +----------+ +----------+
</font>
</pre>
<p>A dual channel host adapter should have two complete ATA
host adapters with no shared logic, no shared signals, no
shared functions of any kind.</p>
<p>According to some estimates, up to 30 percent of all dual
channel host adapters now on the market (as boards or as on
motherboard designs) have a serious flaw. This flaw can
result in data corruption. The flaw is that these dual
channel host adapters "share" the data bus between
the two ATA cables. This results in a host adapter that looks
like this internally:</p>
<pre><font face="Courier New">
ATA interfaces
|
host |
bus V
|| +-----------+ control +----------+ +----------+
|| | | signals | | | |
|| | primary |<----------->| |<-->| |
|| | host | | drive | | drive |
|| | adapter | | 0 | | 1 |
|| | logic | | (master) | | (slave) |
|| | | +-------->| |<-->| |
|| | shared | | | | | |
|| | data | | 16-bit +----------+ +----------+
||<==>| bus |<--+ data
|| | logic | | bus +----------+ +----------+
|| | | | | | | |
|| | | +-------->| |<-->| |
|| | secondary | | drive | | drive |
|| | host | control | 0 | | 1 |
|| | adapter | signals | (master) | | (slave) |
|| | logic |<----------->| |<-->| |
|| | | | | | |
|| +-----------+ +----------+ +----------+
</font>
</pre>
<p>Data corruption can come from two sources in this
design:</p>
<ol>
<li>the minor reason is that the data bus exceeds the ATA 18
inch maximum length. This design makes the two cables
look like one 36 inch cable.</li>
<li>the major reason is that a multitasking operating system
expects to be able to perform an I/O operation at the
same time on both the primary and secondary host
adapters. This design will corrupt data (or cause hang
conditions) because of the shared data bus. </li>
</ol>
<p><b>How can you identify a flawed dual channel host adapter?</b></p>
<p>One way is to look at the printed circuit board and count
the number of direct connections between the pins of the
primary host adapter connector and secondary host adapter
connector. This may be difficult on multilayer printed
circuit boards. It is normal for a few signals (such as
ground signals) to be tied together directly. However, if you
see a large number of pins (more than 16) tied directly
together by copper traces on the circuit board, you are
looking at one of these flawed host adapters.</p>
<p>For example:</p>
<pre><font face="Courier New">
primary secondary
connection connector
pins pins
o o o o pins
\_______________/ \___ tied
pins o o o o together
not _____________________/
tied o o o o
together ___/
... ...
o o o o
</font>
</pre>
<p>However, beware, this visual check is not foolproof! Pins
that don’t appear to be tied together in the area of the
connectors could be tied together at some other location.</p>
<p>My advice: Talk to the host adapter (or motherboard)
technical support people and ask them if the two host adapters
share any signals, logic or functions. Any sharing of data
bus signals, host adapter logic or functions (especially data
transfer logic) would indicated a flawed design that could
corrupt your data.</p>
<p><b>BUYER BEWARE!</b></p>
<p>If you run Linux, check the latest Linux IDE driver
information for a version of the IDE driver that will
serialize all I/O to the two host adapters in order to prevent
strange things.</p>
<p>It is unclear at this time (May95) how WinNT, Win95 or OS/2
will deal with these host adapters.</p>
<p><b>BEWARE OF INTEL TRITON DUAL CHANNEL HOST ADAPTERS</b></p>
<p>The Triton is even worse than first look gave. True, it
does not corrupt data or do strange things with interrupts,
but it can have what I consider to be serious performance
problems. Now that Intel has made the chip spec public, we
can all find out that not only does the chip share the two ATA
data buses but that combined bus is also shared with the ISA
address/data bus function that is also in the chip. Intel
claims "fair round robin" sharing of all the uses of
the single bus. So if you have your serial or parallel ports
on the Triton ISA bus and you have any COM or LPT activity
going on this will be multiplexed with your two ATA interfaces
on the same set of signals comming out of the Triton chip. So
much for high performance.</p>
<h3><a name="T20"><font color="green">Why Disk Drives Break</font></a></h3>
<p><b>Why do hard drives fail?</b></p>
<p>There are three major (and these really are the only three
real reasons hard disk drives fail):</p>
<ol>
<li>they get to hot (your system cooling fan quits working).</li>
<li>they are mishandled (dropped or banged around).<br>
<br>
Drives that have been over heated or mishandled can
develop bad sectors as time goes on. Usually you will see
one bad sector and then a few more and then a bunch more
until the drive is basicly not worth using any more.<br>
<br>
A drive that really gets too hot may refuse to spin up
because the heads get "glued" to the media by
the high heat.</li>
<li>an electronics failure.<br>
<br>
Electronics failures are usually sudden and without
warning. A common time for a drive’s electronics to
fail are on Monday morning after the drive has been
powered off all weekend. It is the heating/cooling cycles
that cause breaks in the printed circuit board or breaks
in the little gold wires inside the chips on the printed
circuit board.<br>
<br>
You can also zap the electronics by handling a drive when
you are not properly grounded.</li>
</ol>
<p><b>What can I do?</b></p>
<p>Make that your drive is properly cooled, don’t drop it
and be grounded before you touch a drive!</p>
<p>OK, every once in awhile a disk vendor will make a bunch of
drives that have some kind of dirt or chemical inside that
should not be there. Drives with this kind of problem usually
don’t last very long and usually fail during the warranty
period. This is a very rare thing to have happen these days
but the failure mode is usually the same as described above
for over heating.</p>
<h3><a name="T22"><font color="green">Plug and Play (PnP)</font></a></h3>
<p>As many people have found out PnP is a joke. It is a cute
marketing thing to make you think that the computer hardware
and software vendors have solved all the hardware and software
configuration problems for you. Just plug in a new device and
the system will recognize it and allow you to use it. Ha
Ha!</p>
<p>What is PnP really? It is a bunch of uncoordinated and
proprietary solutions to specific hardware or software
configuration problems. Many of the so called PnP
specifications were developed by individual companies or
groups of companies (aka, private clubs) to enhanced their
image with the computer buying public. Most of these
specifications are short sighted and don’t address all of
the issues.</p>
<p>There are so many PnP specifications floating around the
industry that no one has a complete list of them. New ones
appear monthly. It is a major mess.</p>
<p>A specific example is the PnP mess related to ATA
(IDE/EIDE) devices. Several years ago various hardware
specificaion groups (VESA and PCI and others) attempted to
defined PnP for various type of host adapters, including ATA
(IDE/EIDE) host adapters. But these groups addressed only the
hardware aspects of the host adapter configuration. They
forgot to address the BIOS issues!</p>
<p>So the PnP mess continues on and grows bigger each day.</p>
<h3><a name="T24"><font color="green">Thermal Calibration and AV Drives</font></a></h3>
<p>Someone once posted this on UseNet:</p>
<p>>The way I see it, there are (at least) two common <br>
>myths floating around about SCSI and EIDE drives.</p>
<p><b>Myth #1: SCSI has better sustained throughput than
EIDE.</b></p>
<p>>As I understand it, the SCSI 3 bus is capable of higher
<br> >bandwidth than EIDE on a PCI bus, but that is
irrelevent,<br> >because the hard drives themselves
don’t come anywhere <br> >near using the available
bandwidth. As far as I can tell,<br> >the high end of SCSI
drives (single drives, anyway) are <br> >capable of approx
7 MB/sec sustained throughput. And<br> >the high-end EIDE
drives (PIO Mode 4) are capable of <br> >about the same
sustained throughput. True or false?</p>
<p>Nearly always FALSE (due to IDE/EIDE’s extremely low
command overhead) except that there are real high capacity and
real high performance drives that come only with the SCSI
interface. This will probably change in the future.</p>
<p>Plus there is the old question of "how many reads are
from the drive’s cache?". A cache read is just a
memory (in the drive) to memory (in the system) transfer and
can, in theory, be done at extremely high speeds (much faster
than the drive’s actually media tranfer rate).</p>
<p>Today, for the same transfer rate, EIDE is cheaper
(especially when you include the cost of host adapters and
cables).</p>
<p>Who knows what tomorrow will bring. There are many people
in the disk drive industry working on faster EIDE and faster
SCSI data transfer protocols. And they probably are also
working on the interface that will replace both EIDE and SCSI
(for disk drives). Neither EIDE or SCSI can keep up with the
disk technology that is just over the hill now and moving
towards us very rapidly. Expect an entirely new hard disk
interface in a few years as drives approach data transfer
rates of 100+MB/second.</p>
<p><b>Myth #2: Only drives sold as "AV" drives are
capable of uninterrupted sustained throughput.</b></p>
<p>This is based on a problem which no longer exists.
<br> >Older drives used to pause every so often to
recalibrate <br> >their head positions, the so-called
"thermal recal". <br> >As I understand it, just
about all drives now use <br> >a system of so-called servo
tracks which are written <br> >onto the platter surfaces.
Thus, as the drives get <br> >hotter and the material
expands, the servo tracks <br> >expand right along with
them, and so there’s no need <br> >for thermal recal
anymore. True or false?</p>
<p>Well, sort of TRUE and sort of FALSE. ALL drives MUST do
thermal calibration every so often.</p>
<p>Until a few years ago most drives had "dedicated
servo" systems (all servo data on one surface and only
read by one head) that required frequent calibration
especially if the drives internal temperature was changing
rapidly (like right after power on). The so called AV
versions of this type of drive attempted to reduce the chance
that the host would notice this drive activity. This activity
can distrub the smooth flow of video or sound data during disk
read/write commands.</p>
<p>Today most all drives use an "embedded servo"
system (servo data mixed in with user data on every data
track, every head reads servo data while reading user data).
While thermal calibration is still required, its impact on the
smooth flow of data can be kept to an absolute mimimum. My
guess is that some disk vendors will still use the AV label
just because it’s a good marketing tool. You may even
see an AV label on an EIDE drive.</p>
<p>However, there is another side of this AV thing that you
didn’t bring up (myth#3?): reduced error recovery.</p>
<p>A true AV drive will implement a set of reduced error
recovery read commands based on the theory that your eye will
notice a pause in the flow of the video data while the drive
attempts its normal full error recovery (the picture will
"pause" or "jump") but your eye will
probably ignore a few bad bits of video data flashing across
the screen (video snow). On course you don’t want your
OS to use this reduced error recovery read command when it is
reading real data (like a directory or you current tax
return’s data!).</p>
<p>The bottom line 99% of the time: Unless you are buying a
drive that will become part of the disk array that is used to
store video data, you probably don’t need an AV drive.
AV drives are really built for this application.</p>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: ATA FAQ</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>ATA FAQ</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<H2><FONT COLOR="blue">Hale's ATA FAQ</FONT></H2>
<p>This is Hale's ATA FAQ, not to be confused with the
Enhanced IDE FAQ by Peter and John.</p>
<p>AND ALSO PLEASE NOTE THAT HALE HAS NOT UPDATED HIS FAQ
RECENTLY SO IT IS A LITTLE OUT OF DATE BUT STILL HAS LOTS OF
GOOD INFORMATION!</p>
<p>First the "legal" stuff...</p>
<ol>
<li>This FAQ is not intended to replace any other FAQ on this
subject but is an attempt to provide historical and technical
information about the ATA interface.</li>
<li>This FAQ is not an endorsement of any vendor’s
product(s).</li>
<li>This FAQ is not a recommendation to purchase any
vendor’s product(s).</li>
<li>Every effort is made to insure that all of the information
presented here is not copyrighted, not proprietary and
unrestricted.</li>
<li>When opinions are stated they are clearly identified,
including the person’s name and email address. Such
opinions are offered as long as they contribute to the
understanding of the subject being discussed. No
"flames" allowed.</li>
</ol>
<p>If you have a question that is not answered here or if you
have unrestricted material that you would like to contribute,
please email it to Hale (see the ATA-ATAPI.COM home page). DO
NOT send material that is copyrighted, proprietary or
otherwise restricted in any way—I can’t use such
material in this document.</p>
<h2><font color="blue">Table of Contents</font></h2>
<ul>
<li><a href="#T3"><font color="green">Glossary</font></a></li>
<li><a href="#T40"><font color="green">Basic Questions</font></a></li>
</ul>
<h2><a name="T3"><font color="blue">Glossary</font></a></h2>
<p>Read and understand these terms. You will be lost and
confused if you don’t! Many of these are describe in
much greater detail in other parts of this FAQ.</p>
<ul>
<li><a href="#T4"><font color="green">ATA or AT Attachment</font></a></li>
<li><a href="#T5"><font color="green">ATA-1</font></a></li>
<li><a href="#T6"><font color="green">ATA-2 or ATA Extensions</font></a></li>
<li><a href="#T7"><font color="green">ATA-3</font></a></li>
<li><a href="#T8"><font color="green">ATAPI or ATA Packet Interface</font></a></li>
<li><a href="#T9"><font color="green">Block Mode</font></a></li>
<li><a href="#T10"><font color="green">CAM (Common Access Method) Committee</font></a></li>
<li><a href="#T11"><font color="green">CHS or Cylinder/Head/Sector</font></a></li>
<li><a href="#T12"><font color="green">Command Block Control Block</font></a></li>
<li><a href="#T13"><font color="green">DMA or Direct Memory Access</font></a></li>
<li><a href="#T14"><font color="green">EIDE or Enhanced IDE</font></a></li>
<li><a href="#T15"><font color="green">ESDI</font></a></li>
<li><a href="#T16"><font color="green">Fast ATA</font></a></li>
<li><a href="#T17"><font color="green">Host or Host System</font></a></li>
<li><a href="#T18"><font color="green">HBA or Host Bus Adapter or Host Adapter</font></a></li>
<li><a href="#T19"><font color="green">IDE</font></a></li>
<li><a href="#T20"><font color="green">LBA or Logical Block Addressing</font></a></li>
<li><a href="#T21"><font color="green">Local Bus</font></a></li>
<li><a href="#T22"><font color="green">Master</font></a></li>
<li><a href="#T23"><font color="green">Megabyte or MB</font></a></li>
<li><a href="#T24"><font color="green">MFM</font></a></li>
<li><a href="#T25"><font color="green">OS</font></a></li>
<li><a href="#T26"><font color="green">PC Card ATA PCMCIA</font></a></li>
<li><a href="#T27"><font color="green">PCI</font></a></li>
<li><a href="#T28"><font color="green">PIO or Programmed Input/Output</font></a></li>
<li><a href="#T29"><font color="green">RLL</font></a></li>
<li><a href="#T30"><font color="green">Slave</font></a></li>
<li><a href="#T31"><font color="green">Task File</font></a></li>
<li><a href="#T32"><font color="green">SCSI</font></a></li>
<li><a href="#T33"><font color="green">SFF or Small Form Factor</font></a></li>
<li><a href="#T34"><font color="green">ST506 and ST412</font></a></li>
<li><a href="#T35"><font color="green">VESA and VL-Bus</font></a></li>
<li><a href="#T36"><font color="green">WG or Working Group</font></a></li>
<li><a href="#T37"><font color="green">X3T9 and X3T10</font></a></li>
<li><a href="#T38"><font color="green">528MB</font></a></li>
<li><a href="#T39"><font color="green">68-pin ATA</font></a></li>
</ul>
<h3><a name="T4"><font color="green">ATA or AT Attachment</font></a></h3>
<p>ATA is the proper and correct name for what most people
call IDE. In this document, ATA refers to all forms of ATA
(ATA-1, ATA-2, etc, IDE, EIDE, etc). The ATA interface uses a
single 40-conductor cable in most desktop systems.</p>
<h3><a name="T5"><font color="green">ATA-1</font></a></h3>
<p>ATA-1 is the common name of the original ATA (IDE)
specification. ATA-1 is not an official standard yet. Final
approval is pending.</p>
<h3><a name="T6"><font color="green">ATA-2 or ATA Extensions</font></a></h3>
<p>ATA-2 is the common name of the new ATA specification.
ATA-2 is still in early draft form and has not been submitted
for approval as an official standard.</p>
<h3><a name="T7"><font color="green">ATA-3</font></a></h3>
<p>ATA-3 is the common name of a future version of the ATA
specification. The ATA-3 working group has held several
meeting but the only things adopted so far are a DMA version
of the Identify command, a description of "device 1 only
configurations" and a set of "security"
commands.</p>
<p>There is much discussion going on concerning merging ATA-3
with ATAPI. This will require some kind of "command
overlap" capability. The details of this are consumming
much of the meeting time.</p>
<h3><a name="T8"><font color="green">ATAPI or ATA Packet
Interface</font></a></h3>
<p>ATAPI is a proposed new interface specification. Initially
it will probably be used for CD-ROM and tape devices. It uses
the ATA hardware interface at the physical level but uses a
subset of the SCSI command set at the logical level. The
ATAPI specification work is currently being done in the SFF
committee.</p>
<p>The ATAPI folks have delayed forwarding their
CD-ROMspecification from SFF to X3T10 so the X3T10 ATAPI
working group has nothing to work on yet and have held no
meetings.</p>
<h3><a name="T9"><font color="green">Block Mode</font></a></h3>
<p>Block mode is the name given to the use of the ATA Read
Multiple and Write Multiple commands. These commands generate
a single interrupt to the host system for each block of
sectors transfered. The traditional Read Sectors and Write
Sectors commands generate an interrupt to the host for each
sector transfered.</p>
<h3><a name="T10"><font color="green">CAM (Common Access
Method) Committee</font></a></h3>
<p>The Common Access Method committee, now disbanded, worked
on two specifications: the CAM SCSI and CAM ATA
specifications. Both specifications were forwarded to the
X3T9 committee for further work years ago.</p>
<h3><a name="T11"><font color="green">CHS or
Cylinder/Head/Sector</font></a></h3>
<p>CHS is the old and traditional way to address data sectors
on a hard disk. This style of addressing relates a
sector’s address to the position of the read/write heads.
In today’s ATA devices, all sector addresses used by the
host are logical and have nothing to do with the actual
physical position of the sector on the media or the actual
position of the read/write heads.</p>
<h3><a name="T12"><font color="green">Command Block
<br>Control Block</font></a></h3>
<p>These are names given to the I/O register interface used by
ATA devices. It refers to a set of I/O registers, or I/O
ports and I/O port addresses used to program the device.
These names replace the older term Task File.</p>
<h3><a name="T13"><font color="green">DMA or Direct Memory
Access</font></a></h3>
<p>DMA is a method of data transfer between two devices that
does not use the system’s main processor as part of the
data path. DMA requires lots of hardware: a DMA arbitration
unit, a DMA data transfer unit and host bus signals that
enable the DMA controller to assume control of the host
system’s bus. When the DMA controller has control of the
host system’s bus, it moves data between the two devices
by generating the appropriate bus read/write cycles. For the
ATA READ DMA command this means generating an I/O read cycle
and then a memory write cycle for each 16-bit word being
transferred. For the ATA WRITE DMA command, a memory read
cycle is followed by an I/O write cycle for each 16-bit word
transferred.</p>
<h3><a name="T14"><font color="green">EIDE or Enhanced IDE</font></a></h3>
<p>EIDE is a marketing program started by Western Digital to
promote certain ATA-2 features including ATAPI. WD has
encouraged other product vendors to mark their products as
"EIDE compatible" or "EIDE capable".</p>
<h3><a name="T15"><font color="green">ESDI</font></a></h3>
<p>See MFM.</p>
<h3><a name="T16"><font color="green">Fast ATA</font></a></h3>
<p>Fast ATA is a Seagate marketing program used to promote
certain ATA-2 features in newer ATA devices. Seagate has
encouraged other product vendors to mark their products as
"Fast ATA compatible" or "Fast ATA
capable".</p>
<h3><a name="T17"><font color="green">Host or Host System</font></a></h3>
<p>The computer system that the ATA device is attached to.</p>
<h3><a name="T18"><font color="green">HBA or Host Bus Adapter
or Host Adapter</font></a></h3>
<p>The hardware that converts host bus signals to/from ATA
interface signals. An ATA-1 host adapter is generally a very
simple piece of hardware. An ATA-2 host adapter can be simple
or complex.</p>
<h3><a name="T19"><font color="green">IDE</font></a></h3>
<p>IDE can mean any number of things: Imbedded Device (or
Drive) Electronics (yes, you can spell embedded with an
"i"), Intelligent Device (or Drive) Electronics,
etc. The term IDE is the trademark of someone (Western
Digital does not claim IDE as theirs but they do claim EIDE).
Many hard disk vendors do not use IDE to describe their
products to avoid any trademark conflicts.</p>
<h3><a name="T20"><font color="green">LBA or Logical Block
Addressing</font></a></h3>
<p>LBA is a newer (for ATA it is newer) way to address data
sectors on a hard disc. This style of addressing uses a
28-bit binary number to address a sector. LBA numbers start
at zero. In today’s ATA devices, all sector addresses
used by the host are logical and have nothing to do with the
actual physical location of the sector on the media.</p>
<h3><a name="T21"><font color="green">Local Bus</font></a></h3>
<p>Usually this refers to the processor’s local bus in a
high performance computer system. Usually the processor, the
external processor instruction/data cache, the main memory
controller and the bridge controller for the next low speed
system bus, for example, a PCI bus, are located on the local
bus. Lower speed local buses may have connectors that allow
the attachment of other devices. For example, the VL-Bus is a
local bus that can allow attachment of video, SCSI or ATA
controllers. It is very difficult to attach other devices to
high speed (say faster than 100MHz) local buses due to
electrical restrictions that come into play at those higher
speeds.</p>
<h3><a name="T22"><font color="green">Master</font></a></h3>
<p>ATA device 0. Device 0, the master, is the
"master" of nothing. See Slave.</p>
<h3><a name="T23"><font color="green">Megabyte or MB</font></a></h3>
<p>Megabyte or MB is 1,000,000 bytes or 10^6 bytes. IT IS NOT
1,048,576 bytes or 2^20 bytes, repeat NOT!</p>
<h3><a name="T24"><font color="green">MFM</font></a></h3>
<p>In this document MFM refers to any of the older hard disk
controller interfaces, MFM, RLL and ESDI. It is used to
describe any hard disk controller that uses the Task File
interface on the host side and the ST506/ST412 interface on
the drive side.</p>
<h3><a name="T25"><font color="green">OS</font></a></h3>
<p>Operating System.</p>
<h3><a name="T26"><font color="green">PC Card ATA
<br>PCMCIA</font></a></h3>
<p>We can thank the Personal Computer Memory Card
International Association for the PC Card specification. The
PCMCIA is a nonprofit industry association. The PC Card ATA
specification is another form of the ATA interface used by
PCMCIA compatible ATA devices. This interface uses the PCMCIA
68-pin connector. Most 68-pin ATA devices are dual mode—
they can operate as either a PCMCIA PC Card ATA device or as a
68-pin ATA device.</p>
<h3><a name="T27"><font color="green">PCI</font></a></h3>
<p>We can thank Intel and the other members of the PCI
committee for the PCI bus specification. PCI is intended to
be the next high performance computer bus. PCI is not
generally described as a processor local bus.</p>
<h3><a name="T28"><font color="green">PIO or Programmed
Input/Output</font></a></h3>
<p>PIO is a method of data transfer between two devices that
uses the system’s main processor as part of the data
path. On x86 systems, the REP INS and REP OUT instructions
implement this data transfer method. INS reads and I/O port
and writes the data into memory. OUTS reads data from memory
and writes the data to an I/O port. Each time an INS or OUTS
instruction is executed, the memory address is updated. The
REP prefix causes the instructions to be repeated until a
counter reaches zero.</p>
<h3><a name="T29"><font color="green">RLL</font></a></h3>
<p>See MFM.</p>
<h3><a name="T30"><font color="green">Slave</font></a></h3>
<p>ATA device 1. Device 1, the slave, is a "slave"
to nothing.</p>
<p>See Master.</p>
<h3><a name="T31"><font color="green">Task File</font></a></h3>
<p>This is the name given to the I/O register interface used
by MFM controllers. It refers to a set of I/O registers, or
I/O ports and I/O port addresses used to program the
controller. In ATA, this name has been replaced by the terms
Command Block and Control Block.</p>
<h3><a name="T32"><font color="green">SCSI</font></a></h3>
<p>See the SCSI FAQ.</p>
<h3><a name="T33"><font color="green">SFF or Small Form Factor</font></a></h3>
<p>The SFF committee is an ad hoc committee formed by most of
the major storage device and system vendors to set standards
for the physical layout of hard disk and other devices. SFF
has published many specifications that describe the physical
mounting and connector specifications for hard disk devices,
including ATA devices. During a brief period of time when the
X3T9 committee was not doing much work on the ATA-1 interface,
the SFF committee published several specifications that were
not really part of the original SFF charter. Most, if not
all, of these nonphysical specifications have now been
incorporated into the latest X3T9 or X3T10 ATA specifications.
ATAPI is currently an SFF specification.</p>
<h3><a name="T34"><font color="green">ST506 and ST412</font></a></h3>
<p>This is the common name for the hard disk controller to
hard disk drive interface used by MFM, RLL and ESDI
controllers and disk drives. ST stands for Seagate
Technology. The ST506 and ST412 were the Seagate products
that set the de facto standards for this interface many years
ago. This interface is composed of two cables: a
34-conductor and a 20-conductor cable.</p>
<h3><a name="T35"><font color="green">VESA and VL-Bus</font></a></h3>
<p>We can thank the Video Electronics Standards Association
for the VESA Local Bus or VL-Bus specification. The VL-Bus is
one example of a local bus. VESA is a nonprofit industry
association like the PCMCIA.</p>
<h3><a name="T36"><font color="green">WG or Working Group</font></a></h3>
<p>The actual work on various specifications and standards
documents within the X3T9, X3T10 and SFF committees happens in
working group meetings. Most WG meetings are held
monthly.</p>
<h3><a name="T37"><font color="green">X3T9 and X3T10</font></a></h3>
<p>These are the names of the official standards committees
that have worked on the ATA-1 and ATA-2 specifications. X3T9
was responsible for the SCSI and ATA-1 specifications and
standards. X3T10 has replaced X3T9 and is now responsible for
the current SCSI and ATA specifications and standards
work.</p>
<h3><a name="T38"><font color="green">528MB</font></a></h3>
<p>This term is used in this document to describe the capacity
boundary that exists in most x86 system software. This
boundary limits the size of an ATA disk drive to 528MB. For
cylinder/head/sector style addressing of disk data sectors,
this number is computed as follows:</p>
<ol>
<li>the number of cylinders are limited to 1024, numbered
0-1023.</li>
<li>the number of heads (per cylinder) are
limited to 16, numbered 0-15,</li>
<li>the number of sectors
(per track) are limited to 63, numbered 1-63.</li>
<li>a sector is 512 bytes,</li> <li>528MB means the following
values:
<br> ( 1024 * 16 * 63 ) or 1,032,192 data sectors
<br>or
<br> ( 1024 * 16 * 63 * 512 ) or 528,482,304 bytes.</li>
</ol>
<h3><a name="T39"><font color="green">68-pin ATA</font></a></h3>
<p>This refers to a variation of the ATA interface that uses
the PCMCIA 68-pin physical interface but does not use the
PCMCIA electrical or logical interface. Most 68-pin ATA
devices are dual mode—they can operate as either a PCMCIA
PC Card ATA device or as a 68-pin ATA device. This interface
was developed within the SFF committee and is now included in
ATA-2.</p>
<h2><a name="T40"><font color="blue">Basic Questions</font></a></h2>
<ul>
<li><a href="#T41"><font color="green">Where did ATA come from?</font></a></li>
<li><a href="#T42"><font color="green">How is ATA different from MFM?</font></a></li>
<li><a href="#T43"><font color="green">Why is ATA so popular?</font></a></li>
<li><a href="#T44"><font color="green">What are the basics of the ATA interface?</font></a></li>
<li><a href="#T45"><font color="green">What is EIDE or Fast ATA?</font></a></li>
<li><a href="#T47"><font color="green">What does an ATA-1 host adapter do?</font></a></li>
<li><a href="#T48"><font color="green">What does an ATA-2 host adapter do?</font></a></li>
<li><a href="#T49"><font color="green">Can I put an ATA-2 device on an ATA-1 host adapter? Can
I put an ATA-1 device on an ATA-2 host adapter?</font></a></li>
<li><a href="#T50"><font color="green">I have an ATA-2 host adapter with an ATA-2 device. I
want to add an ATA-1 device to this host adapter. Can I run the ATA-2 device in its ATA-2
data transfer modes?</font></a></li>
<li><a href="#T51"><font color="green">How many disk controllers and/or ATA host adapters
and/or SCSI host adapters can I put in my system?</font></a></li>
<li><a href="#T52"><font color="green">Are disk drives the only ATA devices?</font></a></li>
<li><a href="#T53"><font color="green">What can be done to improve ATA device performance?</font></a></li>
<li><a href="#T54"><font color="green">What else can be done to improve ATA device
performance? -or- What is PIO mode "x" ?</font></a></li>
<li><a href="#T55"><font color="green">Do I need BIOS or OS drivers to support more than
528MB?</font></a></li>
<li><a href="#T57"><font color="green">Do I need BIOS or OS drivers to support the ATA-2 data
transfer rates?</font></a></li>
<li><a href="#T59"><font color="green">I just purchased a new high speed host adapter for my
VL-Bus (or PCI bus) system and a new 540MB hard disk. How do I get full use out of all
this new hardware?</font></a></li>
</ul>
<h3><a name="T41"><font color="green">Where did ATA come from?</font></a></h3>
<p>What we now call the ATA-1 interface was developed for
Compaq many years ago by Imprimus (then part of CDC, now part
of Seagate) and Western Digital. The first ATA-1 hard disk
drives were made by Imprimus but it was Conner that made the
interface so popular.</p>
<h3><a name="T42"><font color="green">How is ATA different from
MFM?</font></a></h3>
<p>From the host software standpoint, ATA is very much like
the Task File interface used by MFM controllers. A properly
written host software driver should not notice any difference
between the MFM Task File interface and the ATA Command Block
interface while doing basic commands such as Read/Write
Sectors.</p>
<p>At the hardware level, ATA uses a single cable between a
host bus adapter and the ATA device, where the MFM controller
interface uses two cables between the controller and the
drive.</p>
<p>In the MFM environment, the controller is one piece of
hardware and the drive another piece of hardware. Most likely
these two pieces of hardware are from different vendors. The
MFM controller is dependent on the design of both the host bus
and on the drive.</p>
<p>In the ATA environment, the host adapter is the one piece
of hardware that is dependent on the host system bus design.
The ATA interface is (mostly) system independent. All of the
hard disk controller and drive logic is contained in the ATA
device hardware. This gives the hard disk designer complete
control over both the controller and drive functions.</p>
<h3><a name="T43"><font color="green">Why is ATA so popular?</font></a></h3>
<p>Two basic things make ATA so popular today: cost and hard
disk drive technology. An ATA-1 host adapter is cheap,
usually much less than $25US and it uses only one cable. On
the technology side, current hard disk features, such as,
defect handling, error recovery, zone recording, cache
management and power management require that the controller be
fully integrated with the read/write channel, the servo system
and spindle hardware of the disk drive.</p>
<h3><a name="T44"><font color="green">What are the basics of
the ATA interface?</font></a></h3>
<p>The ATA interface is a very simple interface based on an
ISA bus I/O device architecture. The interface consists of
two sets of I/O registers, mostly 8-bit, for passing command
and status information. The registers are like a set of mail
boxes with a door on front and back connected such that both
doors can not be open at the same time. The front door is
open when the Busy bit in the Status register is zero and the
host can read and write the registers. The back door is open
when the Busy bit in the Status register is one and the ATA
device can read or write the registers.</p>
<p>The physical interface contains just enough signals for a
16 bit data bus, five register address bits, and a few control
signals like read register, write register and reset.</p>
<p>ATA devices look like traditional hard disk drives even
though some are not really a hard disc with rotating platters.
User data is recorded in 512 byte sectors. Each sector has a
sector address. There are two ways to express sector
addresses: by cylinder/head/sector (CHS) or by logical block
address (LBA). CHS is standard, LBA is optional.</p>
<h3><a name="T45"><font color="green">What is EIDE or Fast ATA?</font></a></h3>
<p>Both are marketing programs used to promote various ATA-2
features, mostly the faster data transfer rates defined by
ATA-2.</p>
<p>WD defines EIDE as:
<ul>
<li>Support for drives larger than 528MB.</li>
<li>Support for two connectors to allow up to four drives.</li>
<li>Support for CD-ROM and tape peripherals.</li>
<li>Support for 11.1/16.6 Mbytes/second, I/O Channel Ready PIO data transfers.</li>
<li>Support for 13.3/16.6 Mbytes/second, DMA data transfers.</li>
</ul>
<p>Seagate defines Fast ATA as:
<ul>
<li>Support for PIO mode 3 (11.1 MB/sec) and DMA mode 1(13.3 MB/sec).</li>
<li>Support for Multi-sector [Read/Write Multiple] transfers.</li>
<li>Support for >528 MB.</li>
<li>Support for Identify Drive Extensions & Set Transfer Mode Extensions.</li>
<li>Backward compatibility with ATA-1.</li>
</ul>
<p><b>What does all of this mean to us?</b></p>
<p>Support for the ATA-2 high speed PIO and DMA data transfer
modes is both a hardware and software issue.</p>
<p>Support for more than one hard disc controller (or ATA host
adapter) requires the BIOS and/or the operating system to
support more than one Task File or Command/Control Block
register set on the host bus.</p>
<p>The 528MB problem is due to the original design of the x86
BIOS which limits cylinders to 1024 and sectors to 63. The
ATA interface allows up to 65,535 cylinders, 16 heads and 255
sectors—that’s about 136GB (137GB if is LBA is
used). Support for devices over 528MB requires the BIOS
and/or operating system to support some form of CHS
translation. Note that LBA alone does not solve this problem
(in fact, LBA may make things more complex).</p>
<p>Support for CD-ROM and tape will probably be done via the
ATAPI interface which uses a different command structure than
ATA. That makes ATAPI another host software issue.</p>
<h3><a name="T47"><font color="green">What does an ATA-1 host
adapter do?</font></a></h3>
<p>An ATA-1 host adapter is a simple piece of logic whose main
purpose is to reduce the system bus address lines from 12 (or
more) down to 5. It may also buffer some signals giving some
degree of electrical isolation between the host bus (usually
an ISA or EISA bus) and the ATA bus. In ATA-1, the ATA
interface is controlled directly by the host bus so that all
timings are controlled by the host bus timing.</p>
<h3><a name="T48"><font color="green">What does an ATA-2 host
adapter do?</font></a></h3>
<p>This answer is complex because it depends on how smart your
ATA-2 host adapter is. First, an ATA-2 host adapter supports
the ATA-2 higher speed data transfer rates. That requires
that the host adapter is attached to something other than an
ISA or EISA bus. Second, an ATA-2 host adapter may perform
32-bit wide transfers on the host bus. This requires FIFO
logic and data buffers in the host adapter. Third, an ATA-2
host adapter may use a different data transfer protocol on the
host side than is used on the ATA device side.</p>
<h3><a name="T49"><font color="green">Can I put an ATA-2 device
on an ATA-1 host adapter?<br>
Can I put an ATA-1 device on an ATA-2 host adapter?</font></a></h3>
<p>The answer to both questions is yes, as long as the
electrical timing specifications of the device are not
violated. In general it is impossible for an ATA-1 host
adapter to violate the specifications of an ATA-2 device. It
is possible for an ATA-2 host adapter to violate the timing
specifications of an ATA-1 device but this is not common.
Hoever, host adapter hardware design errors or software driver
bugs can cause such a problem. The result will be corrupted
data read or written to the ATA-1 device.</p>
<h3><a name="T50"><font color="green">I have an ATA-2 host
adapter with an ATA-2 device. I want to add an ATA-1 device to this host adapter. Can I
run the ATA-2 device in its ATA-2 data transfer modes?</font></a></h3>
<p>Sorry, <b>NO</b> you can <b>NOT</b> run the new drive in
its faster data transfer modes. Be very careful, most of the
ATA-2 host adapter vendors don’t have anything in their
setup documentation or software to prevent this sort of
thing.</p>
<p>When you run the new drive at a data transfer speed that is
faster than the older drive can support, you are violating the
electrical interface setup and hold times on the older drive.
There is no telling what the older drive will do about this,
but you are asking for data corruption and other nasty
problems on your older drive.</p>
<h3><a name="T51"><font color="green">How many disk controllers
and/or ATA host adapters and/or SCSI host adapters can I put in my system?</font></a></h3>
<p>From a hardware standpoint—as many as you want as long
as there are no I/O port address, memory address or interrupt
request signal conflicts. From a software standpoint it is a
whole different story.</p>
<p>First the simple x86 system hard disk controller
configurations...
<ol>
<li>1 ATA with 1 or 2 drives at I/O port addresses 1Fxh/3Fxh using interrupt request 14
(IRQ14).</li>
<li>1 ATA with 1 drive at I/O port addresses 1Fxh/3Fxh using interrupt request 14 (IRQ14)
plus a SCSI with 1 drive.</li>
<li>1 SCSI with 1 or 2 drives.</li>
</ol>
<p>Other configurations are possible but are most likely not
supported in the system or SCSI host adapter BIOS. And if its
not supported at the BIOS level, it is unlikely to be
supported by an operating system, especially DOS. The primary
reason the above configurations are so restrictive is that the
original IBM x86 BIOS supported only one MFM controller with a
maximum of 2 drives. This restriction was then coded into
much x86 software including many early version of DOS. The
configurations above work because they don’t break this
old rule.</p>
<p>Just remember this—most systems will always boot from
the first drive on the first controller. In a) that is ATA
drive 0, in b) that is ATA drive 0, in c) that is SCSI drive
0.</p>
<p>And now the more complex configurations...</p>
<p>Once you go beyond the three configurations above all bets
are off. Most likely you will need operating system device
drivers in order to access any drives beyond the first two.
And now your real problems start especially if you like to run
more than one operating system!</p>
<p>If you do run more than one OS, then you need equivalent
drivers for each system if you would like to access all the
drives. Plus it would be nice if all the drivers configured
the drives in the same manner and supported all the possible
partitioning schemes and partition sizes. It would be
especially nice if a driver would not destroy the data in a
partition just because it did not understand the file system
format in that partition.</p>
<p>One of the things EIDE promotes is BIOS support for up to
four ATA devices -- 2 ATA host adapters each with 1 or 2
drives. The first would be at I/O port addresses 1Fxh/3Fxh
using interrupt request 14 (IRQ14) and the second at I/O port
addresses 17xh/37xh using interrupt request 15 (IRQ15).
Acceptance of this configuration has not been spreading like
wild fire through the BIOS world.</p>
<p>Lets look at a two complex configurations...</p>
<ol>
<li>ATA with 2 drives and 1 SCSI with 1 or more drives.<br>
Nice configuration. The ATA drives would be supported by the system BIOS and the SCSI
drives may be, could be, should be, supported by the SCSI host adapter BIOS but probably
not. So in order to use the 2 SCSI drives you probably have to disable the BIOS on the
SCSI card and then load a device driver in CONFIG.SYS. And because the SCSI BIOS is
disabled, you then need a SCSI driver for that other OS you run.</li>
<li>ATA with 1 or 2 drives on each.<br>
Also a nice configuration. But because the system BIOS probably only supports the first
controller address, you’ll need a DOS device driver loaded in CONFIG.SYS in order to
access the drives on the second controller. You’ll need that driver even if there is
only one drive on the first controller. You also need a similar driver to support the
second controller in your other OS.</li>
</ol>
<p>Note: I understand that OS/2 does support both MFM/ATA
controller addresses and does allow up to four drives—I
have not confirmed this for myself.</p>
<h3><a name="T52"><font color="green">Are disk drives the only
ATA devices?</font></a></h3>
<p>No. Over the years there have been ATA tape drives, ATA
CD-ROMS and other strange devices. Most of these are expected
to be added to an existing ATA host adapter as the second
device (device 1) with an existing ATA disk drive (device 0).
In general these require software drivers to operate with your
OS.</p>
<p>Now, we have ATAPI CD-ROM and tape devices that can be
placed on an ATA host adapter. And soon we should see system
motherboard BIOS support for booting from an ATAPI CD-ROM
device. The general idea is that an ATAPI device can coexist
with an ATA device on the same cable.</p>
<h3><a name="T53"><font color="green">What can be done to
improve ATA device performance?</font></a></h3>
<p>A difficult question. But the first step is usually to
reduce the number of interrupts that the host sees during a
read or write command. ATA disk drives have three types of
read/write commands:
<ul>
<li>Read Sectors / Write Sectors—These commands are the old traditional data transfer
commands. These commands generate one interrupt to the host for each sector transfered.
These are PIO data in and PIO data out commands which use the host processor to transfer
the data.</li>
<li>Read Multiple / Write Multiple—These commands where defined in ATA-1 but were not
used very much until recently. These commands generate one interrupt to the host for each
block of sectors transfered. The number of sector per block is generally 4, 8 or 16.
However, when 1 sector per block is used, these commands are the same as the Read/Write
Sectors commands. These are PIO data in and PIO data out commands which use the host
processor to transfer the data.</li>
<li>Read DMA / Write DMA—These commands where defined in ATA-1 but were not used very
much until recently. The main reason for not using them was that x86 system DMA transfer
rates are much slower than PIO. However, these command do generate a single interrupt at
the completion of the command.</li>
</ul>
<p>Now see the next question...</p>
<h3><a name="T54"><font color="green">What else can be done to
improve ATA device performance?
<br>-or-
<br>What is PIO mode "x" ?</font></a></h3>
<p>An even more difficult question. The second step is
usually to increase the rate at which the host transfers
data.</p>
<p>(Ahh... I can see the funny look on your face from here.
You are saying to yourself: "the rate at which the host
transfers data? doesn’t this guy have things
backwards?" Read on...)</p>
<p>The rate at which data is transferred to or from an ATA
device is determined by only one thing: the PIO or DMA cycle
time the host uses. No, the drive does not have much to do
with this! The only requirement is that the host not exceed
the minimum PIO or DMA cycle times that the device supports.
For example, during a PIO read command when the device signals
an interrupt to the host this means that the device is waiting
for the host to read the next sector or block of sectors from
the drive. The host must execute a REP INSW instruction to do
transfer the data. The rate at which the host executes this
instruction determines the PIO cycle time. Technically, for a
read command, the cycle time is the time from the host
assertion of the I/O Read signal to the next time the host
asserts the I/O Read signal.</p>
<p>Be careful when looking at the table below—the data
rate is the data transfer rate achieved while transfering the
sector or block or sectors. It is an "instantanous"
data rate. The overall data transfer rate for a command
includes many time consuming events such as the amount of time
the host requires to process an interrupt. Note that on many
fast ATA drives today, the time it takes the host to process
an interrupt is frequently greater than the time required to
transfer the sector of block of sectors for that interrupt!
It is not uncommon for the host overhead to reduce the data
rate to �or 1/3 of the instantanous rate shown here.</p>
<p>The ATA PIO modes are defined as follows:</p>
<table width="80%" border="1" cellspacing="1" cellpadding="5" align="center">
<tr>
<td width="15%" align="center"><b>PIO mode</b></td>
<td width="20%" align="center"><b>min cycle time</b></td>
<td width="15%" align="center"><b>data rate</b></td>
<td width="50%" align="center"><b>comment</b></td>
</tr>
<tr>
<td width="15%" align="center">0</td>
<td width="20%" align="center">600ns</td>
<td width="15%" align="center">3MB</td>
<td width="50%">the rate at which a system running at 4.77MHZ could execute the REP INSW.</td>
</tr>
<tr>
<td width="15%" align="center">1</td>
<td width="20%" align="center">383ns</td>
<td width="15%" align="center">5MB</td>
<td width="50%">the rate at which a system running at 6MHz could execute the REP INSW.</td>
</tr>
<tr>
<td width="15%" align="center">2</td>
<td width="20%" align="center">240ns</td>
<td width="15%" align="center">8MB</td>
<td width="50%">the rate at which a system running at 8MHz could execute the REP INSW.</td>
</tr>
<tr>
<td width="15%" align="center">3</td>
<td width="20%" align="center">180ns</td>
<td width="15%" align="center">11MB</td>
<td width="50%">requires an ATA-2 host adapter.</td>
</tr>
<tr>
<td width="15%" align="center">4</td>
<td width="20%" align="center">120ns</td>
<td width="15%" align="center">16MB</td>
<td width="50%">requires an ATA-2 host adapter.</td>
</tr>
</table>
<p>The complete description of the PIO (and DMA modes is much
more complex and will be cover in more detail later in this
FAQ.</p>
<h3><a name="T55"><font color="green">Do I need BIOS or OS
drivers to support more than 528MB?</font></a></h3>
<p><b>Warning: Read the previous question before reading this
one.</b></p>
<p>Maybe, probably, yes. The answer to this <b>very</b>
complex and will be discussed in detail in Part 2. Here is the
brief answer...</p>
<p>A traditional x86 system BIOS supports only CHS mode
addressing with cylinders limited to 1024, heads limited to 16
and sectors limited to 63. This allows addressing of drives
up to 528MB. These limitations come from the INT 13
read/write calls that combine a 10-bit cylinder number with a
6-bit sector number into a 16-bit register.</p>
<p>Note that this is entirely a software problem: the ATA
interface supports up to 65,535 cylinders, 16 heads and 255
sectors.</p>
<p>While the head number usually requires only 4-bits, up to 6
or 8 bits are available in the INT 13 interface. This fact
has allowed the SCSI folks to support big drives by increasing
the number of heads above 16. The SCSI host adapter BIOS
converts this "fake" CHS address to a different CHS
or an LBA when it issues a read/write command to the drive.
The following table shows some approximate drives sizes and
the "fake" CHS parameters that you may see from a
SCSI BIOS:</p>
<table width="60%" border="1" cellspacing="1" cellpadding="5" align="center">
<tr>
<td width="25%" align="center"><b>cyl</b></td>
<td width="25%" align="center"><b>head</b></td>
<td width="25%" align="center"><b>sector</b></td>
<td width="25%" align="center"><b>size</b></td>
</tr>
<tr>
<td width="25%" align="center">1024</td>
<td width="25%" align="center">16</td>
<td width="25%" align="center">63</td>
<td width="25%" align="center">512MB</td>
</tr>
<tr>
<td width="25%" align="center">1024</td>
<td width="25%" align="center">32</td>
<td width="25%" align="center">63</td>
<td width="25%" align="center">1GB</td>
</tr>
<tr>
<td width="25%" align="center">512</td>
<td width="25%" align="center">64</td>
<td width="25%" align="center">63</td>
<td width="25%" align="center">1GB</td>
</tr>
<tr>
<td width="25%" align="center">1024</td>
<td width="25%" align="center">64</td>
<td width="25%" align="center">63</td>
<td width="25%" align="center">2GB</td>
</tr>
<tr>
<td width="25%" align="center">1024</td>
<td width="25%" align="center">256</td>
<td width="25%" align="center">63</td>
<td width="25%" align="center">8GB</td>
</tr>
</table>
<p>The last entry represents the largest possible drive that a
traditional INT 13 BIOS can support.</p>
<p>The system BIOS folks <b>must</b> start supporting drives
over 528MB in their BIOS by implementing some type of CHS
translation. To date, few systems have such BIOS. And here
is the bad part: Microsoft says that the BIOS <b>must</b>
support it in order to use it in their OS. The algorithm is
simple (but warning: this is not the complete
algorithm!):</p>
<table width="60%" border="1" cellspacing="1" cellpadding="5" align="center">
<tr>
<td width="30%"><b>INT 13 input</b></font></td>
<td width="30%"><b>action</b></font></td>
<td width="40%"><b>ATA interface</b></font></td>
</tr>
<tr>
<td width="30%">cyl number</td>
<td width="30%">"multiply" by n</font></td>
<td width="40%">modified cyl number</font></td>
</tr>
<tr>
<td width="30%">head number</td>
<td width="30%">"divide" by n</font></td>
<td width="40%">modified head number</font></td>
</tr>
<tr>
<td width="30%">sector number</td>
<td width="30%">nothing</td>
<td width="40%">sector number</td>
</tr>
</table>
<p>The value of n must be selected such that the modified head
number is less than 16.</p>
<p>LBA addressing at the hard disk drive level or at the BIOS
or driver level is another solution. This solution will
probably not be popular for several more years. It requires
that the BIOS people implement a new INT 13 interface, called
the Microsoft/IBM Extensions and that the OS people start
using this new BIOS interface. Few system BIOS support this
alternative interface today. Without this new interface, LBA
support at the hard disk drive level is not required.</p>
<p>So most of us have older systems without much possibility
of getting a BIOS upgrade, so what do we do? Well we must
obtain one of the many driver products that are on the market
that live in one of the disk boot sectors and "take
over" the system BIOS INT 13 with an INT 13 that supports
the translation. The biggest problem with this is that the
replacement INT 13 BIOS must live someplace in memory. For
DOS based systems, it can usually live at the top of the 640K
of memory and DOS is made to think that that part of memory,
usually around 8K bytes, does not exist. But the protected
mode OS’s don’t like this and usually wipe out the
driver when they load their kernel. So if you plan to run
multiple OS’s on your system, buyer beware!</p>
<p>Then there is the Windows problem: the standard FastDisk
driver in Windows does <b>not</b> support such translation
schemes and can not be enabled. So make sure the driver you
purchase also comes with a Windows FastDisk replacement.</p>
<p><b>Buyer beware!</b></p>
<h3><a name="T57"><font color="green">Do I need BIOS or OS
drivers to support the ATA-2 data transfer rates?</font></a></h3>
<p><b>Warning: Read the previous two questions before reading
this one.</b></p>
<p>Maybe, probably, yes. The answer to this very complex and
will be discussed in detail in Part 2. Here is the brief
answer...</p>
<p>If you have a new ATA drive that supports the advanced PIO
or DMA data transfer rates (ATA-2 PIO Mode 3 or 4, or, ATA-2
DMA Mode 1 or 2) then you also must have a new ATA host
adapter that attaches to the VL-Bus or PCI bus or some other
high speed bus (probably a 32-bit bus) in your system. That
host adapter has I/O registers of its own that are used to
control its advanced features. Controlling those advanced
features requires software—either in the system INT 13
BIOS or in a INT 13 BIOS on the host adapter card or in a
driver loaded via the boot record or later by your OS.</p>
<p>Depending on how that host adapter works you may also need
a Windows FastDisk replacement in order to use the high speed
data transfer modes in Windows.</p>
<p><b>Buyer beware!</b></p>
<h3><a name="T59"><font color="green">I just purchased a new
high speed host adapter for my VL-Bus (or PCI bus) system and a new 540MB hard disk. How
do I get full use out of all this new hardware?</font></a></h3>
<p><b>Did you read the previous three questions?</b></p>
<p>You need BIOS or driver software and a Windows FastDisk
replacement. These must support both CHS translation (because
your drive is over 528MB) and the host adapter hardware (to
use the high speed data transfer rates).</p>
<p>Some drivers on the market today use LBA addressing on the
ATA interface to get over 528MB. This may make your disk
partition(s) unreadable by another OS.</p>
<p>Check the hardware and software specifications of the
product before you buy it! Ask lots of questions—you
probably get lots of incorrect or misleading answers—be
prepared for that! If you plan to run something other than
DOS and Windows, especially if you plan a "dual
boot" or "boot manager" environment, be real
careful.</p>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: DOS Floppy Disk Boot Sector</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>DOS Floppy Disk Boot Sector</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">Disassembly of a DOS Floppy Boot Sector</font></h2>
<p>Note: I will leave it to someone else to provide you with
a disassembly of an OS/2 HPFS boot sector, or a Linux boot
sector, or a WinNT boot sector, etc.</p>
<p>This article is a disassembly of a floppy disk boot sector
for a DOS floppy. The boot sector of a floppy disk is located
at cylinder 0, head 0, sector 1. This sector is created by a
floppy disk formating program, such as the DOS FORMAT program.
The boot sector of a FAT hard disk partition has a similar
layout and function. Basically a bootable FAT hard disk
partition looks like a big floppy during the early stages of
the system’s boot processing.</p>
<p>At the completion of your system’s Power On Self Test
(POST), INT 19 is called. Usually INT 19 tries to read a boot
sector from the first floppy drive. If a boot sector is found
on the floppy disk, the that boot sector is read into memory
at location 0000:7C00 and INT 19 jumps to memory location
0000:7C00. However, if no boot sector is found on the first
floppy drive, INT 19 tries to read the MBR from the first hard
drive. If an MBR is found it is read into memory at location
0000:7c00 and INT 19 jumps to memory location 0000:7c00. The
small program in the MBR will attempt to locate an active
(bootable) partition in its partition table. If such a
partition is found, the boot sector of that partition is read
into memory at location 0000:7C00 and the MBR program jumps to
memory location 0000:7C00. Each operating system has its own
boot sector format. The small program in the boot sector must
locate the first part of the operating system’s kernel
loader program (or perhaps the kernel itself or perhaps a
"boot manager program") and read that into
memory.</p>
<p>INT 19 is also called when the CTRL-ALT-DEL keys are used.
On most systems, CTRL-ALT-DEL causes an short version of the
POST to be executed before INT 19 is called.
<ul>
<li><a href="#T2"><font color="green">
Where stuff is</font></a></li>
<li><a href="#T4"><font color="green">
Summary of what this thing does</font></a></li>
<li><a href="#T6"><font color="green">
Entire sector in hex and ASCII</font></a></li>
<li><a href="#T8"><font color="green">
The BPB and other data areas</font></a></li>
<li><a href="#T10"><font color="green">
Disassembly of the boot sector</font></a></li>
</ul>
<h3><a name="T2"><font color="green">
Where stuff is</font></a></h3>
<ul>
<li>The BIOS Parameter Block (BPB) starts at offset 0.</li>
<li>The boot sector program starts at offset 3e.</li>
<li>The messages issued by this program start at offset 19e.</li>
<li>The DOS hidden file names start at offset 1e6.</li>
<li>The boot sector signature is at offset 1fe.</li>
</ul>
<h3><a name="T4"><font color="green">
Summary of what this thing does</font></a></h3>
<ol>
<li>Copy Diskette Parameter Table which is pointed to by INT 1E.</li>
<li>Alter the copy of the Diskette Parameter Table.</li>
<li>Alter INT 1E to point to altered Diskette Parameter Table.</li>
<li>Do INT 13 AH=00, disk reset call.</li>
<li>Compute sector address of root directory.</li>
<li>Read first sector of root directory into 0000:0500.</li>
<li>Confirm that first two directory entries are for IO.SYS and MSDOS.SYS.</li>
<li>Read first 3 sectors of IO.SYS into 0000:0700 (or 0070:0000).</li>
<li>Leave some information in the registers and jump to IO.SYS at 0070:0000.</li>
</ol>
<p>Note: This program uses the CHS based INT 13H AH=02 to
read the FAT root directory and to read the IO.SYS file. If
the drive is >528MB, this CHS must be a translated CHS (or
L-CHS, see my BIOS TYPES document). Except for internal
computations no addresses in LBA form are used, another reason
why LBA doesn’t solve the >528MB problem.</p>
<h3><a name="T6"><font color="green">
Entire sector in hex and ASCII</font></a></h3>
<pre><font face="Courier New">
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 eb3c904d 53444f53 352e3000 02010100 *...MSDOS5.0.....*
000010 02e00040 0bf00900 12000200 00000000 *...@............*
000020 00000000 0000295a 5418264e 4f204e41 *......)ZT..NO NA*
000030 4d452020 20204641 54313220 2020fa33 *ME FAT12 .3*
000040 c08ed0bc 007c1607 bb780036 c5371e56 *.....|...x.6.7.V*
000050 1653bf3e 7cb90b00 fcf3a406 1fc645fe *.S..|.........E.*
000060 0f8b0e18 7c884df9 894702c7 073e7cfb *....|.M..G....|.*
000070 cd137279 33c03906 137c7408 8b0e137c *..ry3.9..|t....|*
000080 890e207c a0107cf7 26167c03 061c7c13 *.. |..|...|...|.*
000090 161e7c03 060e7c83 d200a350 7c891652 *..|...|....P|..R*
0000a0 7ca3497c 89164b7c b82000f7 26117c8b *|.I|..K|. ....|.*
0000b0 1e0b7c03 c348f7f3 0106497c 83164b7c *..|..H....I|..K|*
0000c0 00bb0005 8b16527c a1507ce8 9200721d *......R|.P|...r.*
0000d0 b001e8ac 0072168b fbb90b00 bee67df3 *.....r........}.*
0000e0 a6750a8d 7f20b90b 00f3a674 18be9e7d *.u... .....t...}*
0000f0 e85f0033 c0cd165e 1f8f048f 4402cd19 *._.3...^....D...*
000100 585858eb e88b471a 48488a1e 0d7c32ff *XXX...G.HH...|2.*
000110 f7e30306 497c1316 4b7cbb00 07b90300 *....I|..K|......*
000120 505251e8 3a0072d8 b001e854 00595a58 *PRQ.:.r....T.YZX*
000130 72bb0501 0083d200 031e0b7c e2e28a2e *r..........|....*
000140 157c8a16 247c8b1e 497ca14b 7cea0000 *.|..$|..I|.K|...*
000150 7000ac0a c07429b4 0ebb0700 cd10ebf2 *p....t).........*
000160 3b16187c 7319f736 187cfec2 88164f7c *;..|s..6.|....O|*
000170 33d2f736 1a7c8816 257ca34d 7cf8c3f9 *3..6.|..%|.M|...*
000180 c3b4028b 164d7cb1 06d2e60a 364f7c8b *.....M|.....6O|.*
000190 ca86e98a 16247c8a 36257ccd 13c30d0a *.....$|.6%|.....*
0001a0 4e6f6e2d 53797374 656d2064 69736b20 *Non-System disk *
0001b0 6f722064 69736b20 6572726f 720d0a52 *or disk error..R*
0001c0 65706c61 63652061 6e642070 72657373 *eplace and press*
0001d0 20616e79 206b6579 20776865 6e207265 * any key when re*
0001e0 6164790d 0a00494f 20202020 20205359 *ady...IO SY*
0001f0 534d5344 4f532020 20535953 000055aa *SMSDOS SYS..U.*
</font>
</pre>
<h3><a name="T8"><font color="green">
The BPB and other data areas</font></a></h3>
<p>The first 62 bytes of a boot sector are known as the BIOS
Parameter Block (BPB). Here is the layout of the BPB fields
and the values they are assigned in this boot sector:</p>
<pre><font face="Courier New">
db JMP instruction at 7c00 size 2 = eb3c
db NOP instruction 7c02 1 90
db OEMname 7c03 8 'MSDOS5.0'
dw bytesPerSector 7c0b 2 0200
db sectPerCluster 7c0d 1 01
dw reservedSectors 7c0e 2 0001
db numFAT 7c10 1 02
dw numRootDirEntries 7c11 2 00e0
dw numSectors 7c13 2 0b40 (ignore numSectorsHuge)
db mediaType 7c15 1 f0
dw numFATsectors 7c16 2 0009
dw sectorsPerTrack 7c18 2 0012
dw numHeads 7c1a 2 0002
dd numHiddenSectors 7c1c 4 00000000
dd numSectorsHuge 7c20 4 00000000
db driveNum 7c24 1 00
db reserved 7c25 1 00
db signature 7c26 1 29
dd volumeID 7c27 4 5a541826
db volumeLabel 7c2b 11 'NO NAME '
db fileSysType 7c36 8 'FAT12 '
</font>
</pre>
<p>The first 3 bytes of the BPB are JMP and NOP
instructions.</p>
<pre><font face="Courier New">
0000:7C00 EB3C JMP START
0000:7C02 90 NOP
</font>
</pre>
<p>Rest of the BPB.</p>
<pre><font face="Courier New">
0000:7C00 ......4d 53444f53 352e3000 02010100 * MSDOS5.0.....*
0000:7C10 02e00040 0bf00900 12000200 00000000 *...@............*
0000:7C20 00000000 0000295a 5418264e 4f204e41 *......)ZT.&NO NA*
0000:7C30 4d452020 20204641 54313220 2020.... *ME FAT12 *
</font>
</pre>
<p><b>Now pay attention here...</b></p>
<p>The 11 bytes starting at 0000:7c3e are immediately overlaid
by information copied from another part of memory. That
information is the Diskette Parameter Table. This data is
pointed to by INT 1E. This data is:</p>
<ul>
<li>7c3e = Step rate and head unload time.</li>
<li>7c3f = Head load time and DMA mode flag.</li>
<li>7c40 = Delay for motor turn off.</li>
<li>7c41 = Bytes per sector.</li>
<li>7c42 = Sectors per track.</li>
<li>7c43 = Intersector gap length.</li>
<li>7c44 = Data length.</li>
<li>7c45 = Intersector gap length during format.</li>
<li>7c46 = Format byte value.</li>
<li>7c47 = Head settling time.</li>
<li>7c48 = Delay until motor at normal speed.</li>
</ul>
<p>The 11 bytes starting at 0000:7c49 are also overlaid by the
following data:</p>
<ul>
<li>7c49 - 7c4c = diskette sector address (as LBA) of the data area.</li>
<li>7c4d - 7c4e = cylinder number to read from.</li>
<li>7c4f - 7c4f = sector number to read from.</li>
<li>7c50 - 7c53 = diskette sector address (as LBA) of the root directory.</li>
</ul>
<h3><a name="T10"><font color="green">
Disassembly of the boot sector</font></a></h3>
<pre><font face="Courier New">
START: START OF BOOT SECTOR PROGRAM
0000:7C3E FA CLI interrupts off
0000:7C3F 33C0 XOR AX,AX set AX to zero
0000:7C41 8ED0 MOV SS,AX SS is now zero
0000:7C43 BC007C MOV SP,7C00 SP is now 7c00
0000:7C46 16 PUSH SS also set ES
0000:7C47 07 POP ES to zero
The INT 1E vector is at 0000:0078.
Get the address that the vector points to
into the DS:SI registers.
0000:7C48 BB7800 MOV BX,0078 BX is now 78
0000:7C4B 36 SS:
0000:7C4C C537 LDS SI,[BX] DS:SI is now [0:78]
0000:7C4E 1E PUSH DS save DS:SI --
0000:7C4F 56 PUSH SI saves param tbl addr
0000:7C50 16 PUSH SS save SS:BX --
0000:7C51 53 PUSH BX saves INT 1E address
Move the diskette param table to 0000:7c3e.
0000:7C52 BF3E7C MOV DI,7C3E DI is address of START
0000:7C55 B90B00 MOV CX,000B count is 11
0000:7C58 FC CLD clear direction
0000:7C59 F3 REPZ move the diskette param
0000:7C5A A4 MOVSB table to 0000:7c3e
0000:7C5B 06 PUSH ES also set DS
0000:7C5C 1F POP DS to zero
Alter some of the diskette param table data.
0000:7C5D C645FE0F MOV BYTE PTR [DI-02],0F change head settle time
at 0000:7c47
0000:7C61 8B0E187C MOV CX,[7C18] sectors per track
0000:7C65 884DF9 MOV [DI-07],CL save at 0000:7c42
Change INT 1E so that it points to the
altered Diskette param table at 0000:7c3e.
0000:7C68 894702 MOV [BX+02],AX change INT 1E segment
0000:7C6B C7073E7C MOV WORD PTR [BX],7C3E change INT 1E offset
Call INT 13 with AX=0000, disk reset, so
that the new diskette param table is used.
0000:7C6F FB STI interrupts on
0000:7C70 CD13 INT 13 do diskette reset call
0000:7C72 7279 JB TALK jmp if any error
Detemine the starting sector address of
the root directory as an LBA.
0000:7C74 33C0 XOR AX,AX AX is now zero
0000:7C76 3906137C CMP [7C13],AX number sectros zero?
0000:7C7A 7408 JZ SMALL_DISK yes
0000:7C7C 8B0E137C MOV CX,[7C13] number of sectors
0000:7C80 890E207C MOV [7C20],CX save in huge num sects
SMALL_DISK:
0000:7C84 A0107C MOV AL,[7C10] number of FAT tables
0000:7C87 F726167C MUL WORD PTR [7C16] number of fat sectors
0000:7C8B 03061C7C ADD AX,[7C1C] number of hidden sectors
0000:7C8F 13161E7C ADC DX,[7C1E] number of hidden sectors
0000:7C93 03060E7C ADD AX,[7C0E] number of reserved sectors
0000:7C97 83D200 ADC DX,+00 number of reserved sectors
0000:7C9A A3507C MOV [7C50],AX save start addr
0000:7C9D 8916527C MOV [7C52],DX of root dir (as LBA)
0000:7CA1 A3497C MOV [7C49],AX save start addr
0000:7CA4 89164B7C MOV [7C4B],DX of root dir (as LBA)
Determine sector address of first sector
in the data area as an LBA.
0000:7CA8 B82000 MOV AX,0020 size of a dir entry (32)
0000:7CAB F726117C MUL WORD PTR [7C11] number of root dir entries
0000:7CAF 8B1E0B7C MOV BX,[7C0B] bytes per sector
0000:7CB3 03C3 ADD AX,BX
0000:7CB5 48 DEC AX
0000:7CB6 F7F3 DIV BX
0000:7CB8 0106497C ADD [7C49],AX add to start addr
0000:7CBC 83164B7C00 ADC WORD PTR [7C4B],+00 of root dir (as LBA)
Read the first root dir sector into 0000:0500.
0000:7CC1 BB0005 MOV BX,0500 addr to read into
0000:7CC4 8B16527C MOV DX,[7C52] get start of address
0000:7CC8 A1507C MOV AX,[7C50] of root dir (as LBA)
0000:7CCB E89200 CALL CONVERT call conversion routine
0000:7CCE 721D JB TALK jmp is any error
0000:7CD0 B001 MOV AL,01 read 1 sector
0000:7CD2 E8AC00 CALL READ_SECTORS read 1st root dir sector
0000:7CD5 7216 JB TALK jmp if any error
0000:7CD7 8BFB MOV DI,BX addr of 1st dir entry
0000:7CD9 B90B00 MOV CX,000B count is 11
0000:7CDC BEE67D MOV SI,7DE6 addr of file names
0000:7CDF F3 REPZ is this "IO.SYS"?
0000:7CE0 A6 CMPSB
0000:7CE1 750A JNZ TALK no
0000:7CE3 8D7F20 LEA DI,[BX+20] addr of next dir entry
0000:7CE6 B90B00 MOV CX,000B count is 11
0000:7CE9 F3 REPZ is this "MSDOS.SYS"?
0000:7CEA A6 CMPSB
0000:7CEB 7418 JZ FOUND_FILES they are equal
TALK:
Display "Non-System disk..." message,
wait for user to hit a key, restore
the INT 1E vector and then
call INT 19 to start boot processing
all over again.
0000:7CED BE9E7D MOV SI,7D9E "Non-System disk..."
0000:7CF0 E85F00 CALL MSG_LOOP display message
0000:7CF3 33C0 XOR AX,AX INT 16 function
0000:7CF5 CD16 INT 16 read keyboard
0000:7CF7 5E POP SI get INT 1E vector's
0000:7CF8 1F POP DS address
0000:7CF9 8F04 POP [SI] restore the INT 1E
0000:7CFB 8F4402 POP [SI+02] vector's data
0000:7CFE CD19 INT 19 CALL INT 19 to try again
SETUP_TALK:
0000:7D00 58 POP AX pop junk off stack
0000:7D01 58 POP AX pop junk off stack
0000:7D02 58 POP AX pop junk off stack
0000:7D03 EBE8 JMP TALK now talk to the user
FOUND_FILES:
Compute the sector address of the first
sector of IO.SYS.
0000:7D05 8B471A MOV AX,[BX+1A] get starting cluster num
0000:7D08 48 DEC AX subtract 1
0000:7D09 48 DEC AX subtract 1
0000:7D0A 8A1E0D7C MOV BL,[7C0D] sectors per cluster
0000:7D0E 32FF XOR BH,BH
0000:7D10 F7E3 MUL BX multiply
0000:7D12 0306497C ADD AX,[7C49] add start addr of
0000:7D16 13164B7C ADC DX,[7C4B] root dir (as LBA)
Read IO.SYS into memory at 0000:0700. IO.SYS
is 3 sectors long.
0000:7D1A BB0007 MOV BX,0700 address to read into
0000:7D1D B90300 MOV CX,0003 read 3 sectors
READ_LOOP:
Read the first 3 sectors of IO.SYS
(IO.SYS is much longer than 3 sectors).
0000:7D20 50 PUSH AX save AX
0000:7D21 52 PUSH DX save DX
0000:7D22 51 PUSH CX save CX
0000:7D23 E83A00 CALL CONVERT call conversion routine
0000:7D26 72D8 JB SETUP_TALK jmp if error
0000:7D28 B001 MOV AL,01 read one sector
0000:7D2A E85400 CALL READ_SECTORS read one sector
0000:7D2D 59 POP CX restore CX
0000:7D2E 5A POP DX restore DX
0000:7D2F 58 POP AX restore AX
0000:7D30 72BB JB TALK jmp if any INT 13 error
0000:7D32 050100 ADD AX,0001 add one to the sector addr
0000:7D35 83D200 ADC DX,+00 add one to the sector addr
0000:7D38 031E0B7C ADD BX,[7C0B] incr mem addr by sect size
0000:7D3C E2E2 LOOP READ_LOOP read next sector
Leave information in the AX, BX, CX and DX
registers for IO.SYS to use. Finally,
jump to IO.SYS at 0070:0000.
0000:7D3E 8A2E157C MOV CH,[7C15] media type
0000:7D42 8A16247C MOV DL,[7C24] drive number
0000:7D46 8B1E497C MOV BX,[7C49] get start addr of
0000:7D4A A14B7C MOV AX,[7C4B] root dir (as LBA)
0000:7D4D EA00007000 JMP 0070:0000 JUMP TO 0070:0000
MSG_LOOP:
This routine displays a message using
INT 10 one character at a time.
The message address is in DS:SI.
0000:7D52 AC LODSB get message character
0000:7D53 0AC0 OR AL,AL end of message?
0000:7D55 7429 JZ RETURN jmp if yes
0000:7D57 B40E MOV AH,0E display one character
0000:7D59 BB0700 MOV BX,0007 video attrbiutes
0000:7D5C CD10 INT 10 display one character
0000:7D5E EBF2 JMP MSG_LOOP do again
CONVERT:
This routine
converts a sector address (an LBA) to
a CHS address. The LBA is in DX:AX.
0000:7D60 3B16187C CMP DX,[7C18] hi part of LBA > sectPerTrk?
0000:7D64 7319 JNB SET_CARRY jmp if yes
0000:7D66 F736187C DIV WORD PTR [7C18] div by sectors per track
0000:7D6A FEC2 INC DL add 1 to sector number
0000:7D6C 88164F7C MOV [7C4F],DL save sector number
0000:7D70 33D2 XOR DX,DX zero DX
0000:7D72 F7361A7C DIV WORD PTR [7C1A] div number of heads
0000:7D76 8816257C MOV [7C25],DL save head number
0000:7D7A A34D7C MOV [7C4D],AX save cylinder number
0000:7D7D F8 CLC clear carry
0000:7D7E C3 RET return
SET_CARRY:
0000:7D7F F9 STC set carry
RETURN:
0000:7D80 C3 RET return
READ_SECTORS:
The caller of this routine supplies:
AL = number of sectors to read
ES:BX = memory location to read into
and CHS address to read from in
memory locations 7c25 and 7C4d-7c4f.
0000:7D81 B402 MOV AH,02 INT 13 read sectors
0000:7D83 8B164D7C MOV DX,[7C4D] get cylinder number
0000:7D87 B106 MOV CL,06 shift count
0000:7D89 D2E6 SHL DH,CL shift upper cyl left 6 bits
0000:7D8B 0A364F7C OR DH,[7C4F] or in sector number
0000:7D8F 8BCA MOV CX,DX move to CX
0000:7D91 86E9 XCHG CH,CL CH=cyl lo, CL=cyl hi + sect
0000:7D93 8A16247C MOV DL,[7C24] drive number
0000:7D97 8A36257C MOV DH,[7C25] head number
0000:7D9B CD13 INT 13 read sectors
0000:7D9D C3 RET return
Data not used.
0000:7D90 ca86e98a 16247c8a 36257ccd 13c3.... *.....$|.6%|... *
Messages here.
0000:7D90 ........ ........ ........ ....0d0a * ..*
0000:7Da0 4e6f6e2d 53797374 656d2064 69736b20 *Non-System disk *
0000:7Db0 6f722064 69736b20 6572726f 720d0a52 *or disk error..R*
0000:7Dc0 65706c61 63652061 6e642070 72657373 *eplace and press*
0000:7Dd0 20616e79 206b6579 20776865 6e207265 * any key when re*
0000:7De0 6164790d 0a00.... ........ ........ *ady... *
MS DOS hidden file names (first two root directory entries).
0000:7De0 ........ ....494f 20202020 20205359 * IO SY*
0000:7Df0 534d5344 4f532020 20535953 000055aa *SMSDOS SYS..U.*
The last two bytes contain a 55AAH signature.
0000:7Df0 ........ ........ ........ ....55aa * U.*
</font>
</pre>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: Partition Tables</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>Partition Tables</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">All About Partition Tables and Sectors and FDISK</font></h2>
<p>Here is the most complete description of how partition
tables work.</p>
<ul>
<li><a HREF="#T1"><font color="green">
PARTITION SECTOR/RECORD/TABLE BASICS</font></a></li>
<li><a HREF="#T2"><font color="green">
PARTITION TABLE ENTRY FORMAT</font></a></li>
<li><a HREF="#T3"><font color="green">
PARTITION TABLE RULES</font></a></li>
<li><a HREF="#T4"><font color="green">
PARTITION NESTING</font></a></li>
<li><a HREF="#T5"><font color="green">
PARTITION TABLE LINKING</font></a></li>
<li><a HREF="#T6"><font color="green">
EXAMPLE 1</font></a></li>
<li><a HREF="#T7"><font color="green">
EXAMPLE 2</font></a></li>
<li><a HREF="#T8"><font color="green">
EXAMPLE 3</font></a></li>
<li><a HREF="#T9"><font color="green">
NOTES</font></a></li>
<li><a HREF="#T10"><font color="green">
PARTITION TYPE CODES</font></a></li>
</ul>
<h2><font COLOR="blue"><a NAME="T1">PARTITION SECTOR/RECORD/TABLE BASICS</a></font></h2>
<p>FDISK creates all partition records (sectors). The primary
purpose of a partition record is to hold a partition table.
The rules for how FDISK works are unwritten but so far most
FDISK programs (DOS, OS/2, WinNT, etc) seem to follow the same
basic idea.</p>
<p>First, all partition table records (sectors) have the same
format. This includes the partition table record at cylinder
0, head 0, sector 1 -- what is known as the Master Boot Record
(MBR). The last 66 bytes of a partition table record contain
a partition table and a 2 byte signature. The first 446 bytes
of these sectors usually contain a program but only the
program in the MBR is ever executed (so extended partition
table records could contain something other than a program in
the first 466 bytes). See <a href="hiwmbr.htm"><font
color="green">Master Boot Record</font></a>.</p>
<p>Second, extended partitions are "nested" inside
one another and extended partition table records form a
"linked list". I will attempt to show this in a
diagram below.</p>
<h2><font COLOR="blue"><a NAME="T2">PARTITION TABLE ENTRY FORMAT</a></font></h2>
<p>Each partition table entry is 16 bytes and contains things
like the start and end location of a partition in CHS, the
start in LBA, the size in sectors, the partition
"type" and the "active" flag. Warning:
older versions of FDISK may compute incorrect LBA or size
values. And note: When your computer boots itself, only the
CHS fields of the partition table entries are used (another
reason LBA doesn’t solve the >528MB problem). The CHS
fields in the partition tables are in L-CHS format—see
"How It Works—CHS Translation".</p>
<p>There is no central clearing house to assign the codes used
in the one byte "type" field, however, there is at
least one person at both Microsoft and IBM that attempt to
keep track of the type codes. Type codes are used to define
most every type of file system that anyone has ever
implemented on the x86 PC: 12-bit FAT, 16-bit FAT, HPFS,
NTFS, etc. Plus, an extended partition also has a unique type
code. Refer to <a href="#T10">Partition Type Codes</a> for
the complete list of known partition type codes is contained
in part 2 of this document.</p>
<p>The 16 bytes of a partition table entry are used as
follows:</p>
<pre><font FACE="Courier New">
+--- Bit 7 is the active partition flag, bits 6-0 are
| zero (when not zero this byte is also the drive
| number of the drive to boot so the active
| partition is always found on drive 80H,
| the first hard disk).
|
| +--- Starting CHS in INT 13 call format.
| |
| | +--- Partition type byte.
| | |
| | | +--- Ending CHS in INT 13 call format.
| | | |
| | | | +-- Starting LBA.
| | | | |
| | | | | +-- Size in sectors.
| | | | | |
v ---+---- v ---+---- v v
0. 1. 2. 3. 4. 5. 6. 7. 8.9.A.B. C.D.E.F.
DL DH CL CH TB DH CL CH LBA..... SIZE....
80 01 01 00 06 0e be 94 3e000000 0c610900 1st entry
00 00 81 95 05 0e fe 7d 4a610900 724e0300 2nd entry
00 00 00 00 00 00 00 00 00000000 00000000 3rd entry
00 00 00 00 00 00 00 00 00000000 00000000 4th entry
</font>
</pre>
<p>Bytes 0-3 are used by the small program in the Master Boot
Record to read the first sector of an active partition into
memory. The DH, DL, CH and CL above show which x86 register
is loaded when the MBR program calls INT 13H AH=02H to read
the active partition’s boot sector. See
<a href="hiwmbr.htm"><font color="green">Master Boot Record</font></a>.</p>
<p>These entries define the following partitions:</p>
<ol>
<li>The first partition, a primary partition DOS FAT, starts at CHS 0H,1H,1H (LBA 3EH) and
ends at CHS 294H,EH,3EH with a size of 9610CH sectors.</li>
<li>The second partition, an extended partition, starts at CHS 295H,0H,1H (LBA 9614AH) and
ends at CHS 37DH,EH,3EH with a size of 34E72H sectors.</li>
<li>The third and fourth table entries are unused.</li>
</ol>
<h2><font COLOR="blue"><a NAME="T3">PARTITION TABLE RULES</a></font></h2>
<p>Keep in mind that there are NO written rules and NO
industry standards on how FDISK should work but here are some
basic rules that seem to be followed by most versions of
FDISK:</p>
<ol>
<li>In the MBR there can be 0-4 "primary" partitions, OR, 0-3 primary partitions
and 0-1 extended partition entry.</li>
<li>In an extended partition there can be 0-1 "secondary" partition entries and
0-1 extended partition entries.</li>
<li>Only 1 primary partition in the MBR can be marked "active" at any given time.</li>
<li>In most versions of FDISK, the first sector of a partition will be aligned such that it
is at head 0, sector 1 of a cylinder. This means that there may be unused sectors on the
track(s) prior to the first sector of a partition and that there may be unused sectors
following a partition table sector.<br>
<br>
For example, most new versions of FDISK start the first partition (primary or extended) at
cylinder 0, head 1, sector 1. This leaves the sectors at cylinder 0, head 0, sectors 2...n
as unused sectors. This same layout may be seen on the first track of an extended
partition. See<a href="#T7"> example 2</a> below.<br>
<br>
Also note that software drivers like Ontrack’s Disk Manager depend on these unused
sectors because these drivers will "hide" their code there (in cylinder 0, head
0, sectors 2...n). This is also a good place for boot sector virus programs to hang out.</li>
<li>The partition table entries (slots) can be used in any order. Some versions of
FDISK fill the table from the bottom up and some versions of FDISK fill the table from the
top down. Deleting a partition can leave an unused entry (slot) in the middle of a table.</li>
<li>And then there is the "hack" that some newer OS’s (OS/2 and Linux) use in
order to place a partition spanning or passed cylinder 1024 on a system that does not have
a CHS translating BIOS. These systems create a partition table entry with the
partition’s starting and ending CHS information set to all FFH. The starting and
ending LBA information is used to describe the location of the partition. The LBA can be
converted back to a CHS—most likely a CHS with more than 1024 cylinders. Since such a
CHS can’t be used by the system BIOS, these partitions can not be booted or accessed
until the OS’s kernel and hard disk device drivers are loaded. It is not known if the
systems using this "hack" follow the same rules for the creation of these type
of partitions.</li>
</ol>
<p>There are NO written rules as to how an OS scans the
partition table entries so each OS can have a different
method. For DOS, this means that different versions could
assign different drive letters to the same FAT file system
partitions.</p>
<h2><font COLOR="blue"><a NAME="T4">PARTITION NESTING</a></font></h2>
<p>What do I mean when I say the partitions are
"nested" within each other? Lets look at this
example:</p>
<p>M = Master Boot Record (and any unused sectorson the same
track)</p>
<p>E = Extended partition record (and any unused sectors on
the same track) pri = a primary partition (first sector is a
"boot" sector) sec = a secondary partition (first
sector is a "boot" sector)</p>
<pre><font FACE="Courier New">
|-----------------the entire disk---------------|
| |
|M[pri] |
| E[sec]----rest of 1st ext part-----------|
| E[sec]----rest of 2nd ext part-----|
</font>
</pre>
<p>The first extended partition is described in the MBR and it
occupies the entire disk following the primary partition. The
second extended partition is described in the first extended
partition record and it occupies the entire disk following the
first secondary partition.</p>
<h2><font COLOR="blue"><a NAME="T5">PARTITION TABLE LINKING</a></font></h2>
<p>What do I mean when I say the partition records (tables)
form a "linked" list? This means that the MBR has
an entry that describes (points to) the first extended
partition, the first extended partition table has an entry
that describes (points to) the second extended partition
table, and so on. There is, in theory, no limited to out long
this linked list is. When you ask FDISK to show the DOS
"logical drives" it scans the linked list looking
for all of the DOS FAT type partitions that may exist.
Remember that in an extended partition table, only two entries
of the four can be used (rule 2 above).</p>
<p>And one more thing... Within a partition, the layout of
the file system data varies greatly. However, the first
sector of a partition is expected to be a "boot"
sector. A DOS FAT file system has: a boot sector, first FAT
sectors, second FAT sectors, root directory sectors and
finally the file data area. See
<a href="hiwos2.htm"><font color="green">OS2 Boot Sector</font></a>.</p>
<h2><font COLOR="blue"><a NAME="T6">EXAMPLE 1</a></font></h2>
<p>A disk containing four DOS FAT partitions (C, D, E and
F):</p>
<pre><font FACE="Courier New">
|----------------------the entire disk--------------------|
| |
|M[---C:---] |
| E[---D:---]--rest of 1st ext part-------------|
| E[---E:---]--rest of 2nd ext part--|
| E[---------F:----------]|
</font>
</pre>
<h2><font COLOR="blue"><a NAME="T7">EXAMPLE 2</a></font></h2>
<p>So here is an example of a disk with two primary
partitions, one DOS FAT and one OS/2 HPFS, plus an extended
partition with another DOS FAT:</p>
<pre><font FACE="Courier New">
|-------------------the entire disk-------------------|
| |
|M[pri 1 - DOS FAT] |
| [pri 2 - OS/2 HPFS] |
| E[sec - DOS FAT]|
</font>
</pre>
<p>Or in more detail (‘n’ is the highest cylinder,
head or sector number number allowed in the indicated field of
the CHS)...</p>
<pre><font FACE="Courier New">
+-------------------------------------+
CHS=0,0,1 | Master Boot Record containing |
| partition table search program and |
| a partition table |
| +---------------------------------+ |
| | DOS FAT partition description | | points to CHS=0,1,1
| +---------------------------------+ | points to CHS=a
| | OS/2 HPFS partition description | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | extended partition entry | | points to CHS=b
| +---------------------------------+ |
+-------------------------------------+
CHS=0,0,2 | the rest of "track 0" -- this is | :
to | where the software drivers such as | : normally
CHS=0,0,n | Ontrack's Disk Manager or Micro | : unused
| House's EZ Drive are located. | :
+-------------------------------------+
CHS=0,1,1 | Boot sector for the DOS FAT | :
| partition | : a DOS FAT
+-------------------------------------+ : file
CHS=0,1,2 | rest of the DOS FAT partition | : system
to | (FAT table, root directory and | :
CHS=x-1,n,n | user data area) | :
+-------------------------------------+
CHS=x,0,1 | Boot sector for the OS/2 HPFS | :
| file system partition | : an OS/2
+-------------------------------------+ : HPFS file
CHS=x,0,2 | rest of the OS/2 HPFS file system | : system
to | partition | :
CHS=y-1,n,n | | :
+-------------------------------------+
CHS=y,0,1 | Partition record for the extended |
| partition containing a partition |
| record program (never executed) and |
| a partition table |
| +---------------------------------+ |
| | DOS FAT partition description | | points to CHS=b+1
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
| | unused table entry | |
| +---------------------------------+ |
+-------------------------------------+
CHS=y,0,2 | the rest of the first track of the | : normally
to | extended partition | : unused
CHS=y,0,n | | :
+-------------------------------------+
CHS=y,1,1 | Boot sector for the DOS FAT | :
| partition | : a DOS FAT
+-------------------------------------+ : file
CHS=y,1,2 | rest of the DOS FAT partition | : system
to | (FAT table, root directory and | :
CHS=n,n,n | user data area) | :
+-------------------------------------+
</font>
</pre>
<h2><font COLOR="blue"><a NAME="T8">EXAMPLE 3</a></font></h2>
<p>Here is a partition record from an extended partition (the
first sector of an extended partition). Note that it contains
no program code. It contains only the partition table and the
signature data.</p>
<pre><font FACE="Courier New">
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 00000000 00000000 00000000 00000000 *................*
000010 TO 0001af SAME AS ABOVE
0001b0 00000000 00000000 00000000 00000001 *................*
0001c0 8195060e fe7d3e00 0000344e 03000000 *.....}....4N....*
0001d0 00000000 00000000 00000000 00000000 *................*
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
</font>
</pre>
<h2><font COLOR="blue"><a NAME="T9">NOTES</a></font></h2>
<p>Thanks to Kenneth C. Yue for pointing out that in V0 of
this document I did not properly describe the unused sectors
normally found around the partition table sectors.</p>
<p>Thanks to Marcus Better for pointing out that in V1a-c of
this document I did not properly describe the x86 registers
that the partition table entry data is loaded into when INT 13
is called.</p>
<h2><font COLOR="blue"><a NAME="T10">PARTITION TYPE CODES</a></font></h2>
<p>The following table of partition type codes was compiled
from many sources including information from kind people at
IBM and Microsoft plus Ralf Brown’s list.</p>
<p>Both IBM and Microsoft keep the partition type code lists
and both have until recently assigned type codes. However,
there apparently is no formal agreement between these two
companies and their lists are not always in sync or up to
date. It should be noted that the lists I obtained from IBM
and Microsoft are fairly short and show most of type type
codes as "available" or "reserved". My
guess is that many type codes have been used without the
knowledge of the IBM or Microsoft. This is probably the case
since neither IBM or Microsoft have published a phone number
or email address to contact if you wanted to request a type
code. Chaos is the keyword here.</p>
<p>It now appears that neither company is assigning new type
codes. I have also been told that it is now recommended that
anyone defining a new partition type, or more correctly
stated, anyone defining a new file system type, should use
partition type 07 and use the first block(s) of the partition
to fully define the file system type. Of course, the
appearance of a new type code could cause problems for older
versions of FDISK, various older operating system device
drivers and disk utility programs. (This new use of type code
07 doesn’t explain the recent "assignment" of
type codes 0E and 0F.)</p>
<p>[If you know of a type code assignment that is not listed
here please let me know about it ASAP. Thanks, Hale]</p>
<p>Note that several type codes have multiple uses (for
example, see code 08). Also note that there is some question
about the use of some codes as denoted by a ‘?’ in
the description.</p>
<pre><font FACE="Courier New">
Code Description
00 Unused partition table entry
01 DOS, 12-bit FAT
02 XENIX root
03 XENIX user
04 DOS, 16-bit FAT
05 Extended partition (includes other partition types)
06 DOS, 32+MB support, up to 64K Allocation unit
07 See partition boot record(s) for file system type:
could be QNX, OS/2 HPFS, Windows NT NTFS, Unix, ...
08 OS/2 (thru Version 1.3 only)
08 DELL partition spanning multiple drives (array)
08 Commodore DOS
08 AIX boot? or file system?
09 AIX boot? or file system?
09 Coherent swap
0A OS/2 Boot Manager
0A OPUS
0A Coherent swap
0B FAT32
0C FAT32 LBA (SEE NOTE BELOW)
0D ? (perhaps a type 07 LBA, SEE NOTE BELOW)
0E FAT16 LBA (SEE NOTE BELOW)
0F Extended partition LBA (SEE NOTE BELOW)
NOTE: Partitions types 0C, 0E and 0F (perhaps 0D too)
REQUIRE that the system’s INT 13 BIOS support the
IBM/Microsoft/Phoenix extended/enhanced functions calls
(AH=4x). In these partition table entries the CHS fields are
NOT used and are generally set to maximum values (all 1 bits)
in each CHS field. What this means is that some day there
will be an LBA type partition for all the other partition
types that are listed here and still in use by some system.
This will certainly use up many of the currently unused type
codes!
10 OPUS
11 OS/2 Boot Manager: Inactive type 01
12 Compaq diagnostics
13 Available for assignment
14 OS/2 Boot Manager: Inactive type 04
14 Novell DOS 7.0 FDISK (result of bug in FDISK?)
15 Available for assignment
16 OS/2 Boot Manager: Inactive type 06
17 OS/2 Boot Manager: Inactive type 07
18 AST Windows swap file
18 Compaq System Diagnostics
19 - 1F Available for assignment
20 Available for assignment
21 Reserved
22 Available for assignment
23 Reserved
24 NEC version of MS-DOS
25 Available for assignment
26 Reserved
27 - 2F Available for assignment
30 Available for assignment
31 Reserved
32 Available for assignment
33 Reserved
34 Reserved
35 Available for assignment
36 Reserved
37 - 3B Available for assignment
3C PowerQuest PartitionMagic recovery partition
3D - 3F Available for assignment
40 VENIX :Venix 80286
41 Personal RISC Boot
41 PowerPC boot
42 Secure File System (Peter Gutmann)
43 - 4E Available for assignment
4F Oberon boot and data
50 OnTrack Disk Manager (read-only)
51 OnTrack Disk Manager (write-only)
51 Novell
51 OnTrack Disk Manager (read-only)
52 CP/M
52 Microport
53 OnTrack Disk Manager (write-only)
54 OnTrack Disk Manager (DDO)
55 Available for assignment
56 GoldenBow VFeature
57 - 5F Available for assignment
60 Available for assignment
61 SpeedStor
62 Available for assignment
63 UNIX System V/386
63 Mach, MtXinu BSD 4.3 on Mach
63 GNU HURD
64 Speedstore
64 Novell
65 Novell 286 Netware
66 Novell 386 Netware
67 Novell
68 Novell
69 Novell
6A - 6F Available for assignment
70 DiskSecure Multi-Boot
71 Reserved
72 Available for assignment
73 - 74 Reserved
75 PC/IX
76 Reserved
77 - 79 Available for assignment
7A - 7F ? (probably "available for assignment", these codes
are not shown in the IBM or Microsoft lists!)
80 Minix (ver. 1.4a and earlier)
81 Minix (ver. 1.4b and later)
81 Mitac Advanced Disk Manager
81 Linux
82 Prime
82 Linux swap
82 Solaris UNIX
83 Linux ext2fs
84 OS/2 hiding a type 04
85 Available for assignment
86 Reserved
87 HPFS FT mirrored partition
88 - 8F Available for assignment
90 - 92 Available for assignment
93 Ameba file system
94 Ameba bad block table
95 - 98 Available for assignment
99 Mylex EISA SCSI
9A - 9F Available for assignment
A0 Available for assignment
A1 Reserved
A2 Available for assignment
A3 - A4 Reserved
A5 FreeBSD
A6 Reserved
A7 - A8 Available for assignment
A9 NetBSD
AA - AF Available for assignment
B0 Available for assignment
B1 Reserved
B2 Available for assignment
B3 - B4 Reserved
B5 Available for assignment
B6 Reserved
B7 BSDI file system or secondarily swap
B8 BSDI swap or secondarily file system
B9 - BD Available for assignment
BE Solaris boot partition
BF Available for assignment
C0 Available for assignment
C1 DR-DOS LOGIN.EXE-secured 12-bit FAT
C2 BlueSky Innovations boot manager
C3 BlueSky Innovations boot manager
C4 DR-DOS LOGIN.EXE-secured 16-bit FAT
C5 Available for assignment
C6 DR-DOS LOGIN.EXE-secured Huge
C7 HPFS FT disabled mirrored partition
C7 Cyrnix Boot
C8 - CF Available for assignment
D0 - D7 Available for assignment
D8 CP/M 86
D9 - DA Available for assignment
DB Concurrent DOS, CP/M and CTOS
DC - DF Available for assignment
E0 Available for assignment
E1 Speedstore
E2 Available for assignment
E3 Storage Dimensions
E4 Speedstore
E5 - E6 Reserved
E7 - EA Available for assignment
EB BeOS BFS
EC - EF Available for assignment
F0 Available for assignment
F1 Storage Dimensions
F2 DOS 3.3+ second partition
F3 Reserved
F4 Speedstore
F4 Storage Dimensions
F5 Available for assignment
F6 Reserved
F7 - FD Available for assignment
FE IBM PS/2 IML, LANstep
FF Xenix(?) Bad Block Tables
</font>
</pre>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: OS/2 Boot Sector</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>OS/2 Boot Sector</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">Disassembly of an OS/2 Boot Sector</font></h2>
<p>Note: I will leave it to someone else to provide you with
a disassembly of an OS/2 HPFS boot sector, or a Linux boot
sector, or a WinNT boot sector, etc.</p>
<p>This article is a disassembly of a floppy or hard disk boot
sector for OS/2. Apparently OS/2 uses the same boot sector
for both environments. Basically a bootable FAT hard disk
partition looks like a big floppy during the early stages of
the system’s boot processing. This sector is at cylinder
0, head 0, sector 1 of a floppy or it is the first sector
within a FAT hard disk partition. OS/2 floppy disk and hard
disk boot sectors are created by the OS/2 FORMAT program.</p>
<p>At the completion of your system’s Power On Self Test
(POST), INT 19 is called. Usually INT 19 tries to read a boot
sector from the first floppy drive. If a boot sector is found
on the floppy disk, the that boot sector is read into memory
at location 0000:7C00 and INT 19 jumps to memory location
0000:7C00. However, if no boot sector is found on the first
floppy drive, INT 19 tries to read the MBR from the first hard
drive. If an MBR is found it is read into memory at location
0000:7c00 and INT 19 jumps to memory location 0000:7c00. The
small program in the MBR will attempt to locate an active
(bootable) partition in its partition table. If such a
partition is found, the boot sector of that partition is read
into memory at location 0000:7C00 and the MBR program jumps to
memory location 0000:7C00. Each operating system has its own
boot sector format. The small program in the boot sector must
locate the first part of the operating system’s kernel
loader program (or perhaps the kernel itself or perhaps a
"boot manager program") and read that into
memory.</p>
<p>INT 19 is also called when the CTRL-ALT-DEL keys are used.
On most systems, CTRL-ALT-DEL causes an short version of the
POST to be executed before INT 19 is called.</p>
<ul>
<li><a href="#T2"><font color="green">
Where stuff is</font></a></li>
<li><a href="#T4"><font color="green">
Summary of what this thing does</font></a></li>
<li><a href="#T6"><font color="green">
Entire sector in hex and ASCII</font></a></li>
<li><a href="#T8"><font color="green">
The BPB and other data areas</font></a></li>
<li><a href="#T10"><font color="green">
Disassembly of the boot sector</font></a></li>
</ul>
<h3><a name="T2"><font color="green">
Where stuff is</font></a></h3>
<ul>
<li>The BIOS Parameter Block (BPB) starts at offset 0.</li>
<li>The boot sector program starts at offset 46.</li>
<li>The messages issued by this program start at offset 198.</li>
<li>The OS/2 boot loader file name starts at offset 1d5.</li>
<li>The boot sector signature is at offset 1fe.</li>
</ul>
<h3><a name="T4"><font color="green">
Summary of what this thing does</font></a></h3>
<ol>
<li>If booting from a hard disk partition, skip to step 6.</li>
<li>Copy Diskette Parameter Table which is pointed to by INT 1E to the top of memory.</li>
<li>Alter the copy of the Diskette Parameter Table.</li>
<li>Alter INT 1E to point to altered Diskette Parameter Table at the top of memory.</li>
<li>Do INT 13 AH=00, disk reset call so that the altered Diskette Parameter Table is used.</li>
<li>Compute sector address of the root directory.</li>
<li>Read the entire root directory into memory starting at location 1000:0000.</li>
<li>Search the root directory entires for the file OS2BOOT.</li>
<li>Read the OS2BOOT file into memory at 0800:0000.</li>
<li>Do a far return to enter the OS2BOOT program at 0800:0000.</li>
</ol>
<p>NOTES: This program uses the CHS based INT 13H AH=02 to
read the FAT root directory and to read the OS2BOOT file. If
the drive is >528MB, this CHS must be a translated CHS (or
L-CHS, see my BIOS TYPES document). Except for internal
computations no addresses in LBA form are used, another reason
why LBA doesn’t solve the >528MB problem.</p>
<h3><a name="T6"><font color="green">
Entire sector in hex and ASCII</font></a></h3>
<pre><font face="Courier New">
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 eb449049 424d2032 302e3000 02100100 *.D.IBM 20.0.....*
000010 02000200 00f8d800 3e000e00 3e000000 *................*
000020 06780d00 80002900 1c0c234e 4f204e41 *.x....)...#NO NA*
000030 4d452020 20204641 54202020 20200000 *ME FAT ..*
000040 00100000 0000fa33 db8ed3bc ff7bfbba *.......3.....{..*
000050 c0078eda 803e2400 00753d1e b840008e *......$..u=..@..*
000060 c026ff0e 1300cd12 c1e0068e c033ff33 *.............3.3*
000070 c08ed8c5 367800fc b90b00f3 a41fa118 *....6x..........*
000080 0026a204 001e33c0 8ed8a378 008c067a *......3....x...z*
000090 001f8a16 2400cd13 a0100098 f7261600 *....$...........*
0000a0 03060e00 5091b820 00f72611 008b1e0b *....P.. ........*
0000b0 0003c348 f7f35003 c1a33e00 b800108e *...H..P.........*
0000c0 c033ff59 890e4400 58a34200 33d2e873 *.3.Y..D.X.B.3..s*
0000d0 0033db8b 0e11008b fb51b90b 00bed501 *.3.......Q......*
0000e0 f3a65974 0583c320 e2ede335 268b471c *..Yt... ...5..G.*
0000f0 268b571e f7360b00 fec08ac8 268b571a *..W..6........W.*
000100 4a4aa00d 0032e4f7 e203063e 0083d200 *JJ...2..........*
000110 bb00088e c333ff06 57e82800 8d360b00 *.....3..W.(..6..*
000120 cbbe9801 eb03bead 01e80900 bec201e8 *................*
000130 0300fbeb feac0ac0 7409b40e bb0700cd *........t.......*
000140 10ebf2c3 50525103 061c0013 161e00f7 *....PRQ.........*
000150 361800fe c28ada33 d2f7361a 008afa8b *6......3..6.....*
000160 d0a11800 2ac34050 b402b106 d2e60af3 *....*.@P........*
000170 8bca86e9 8a162400 8af78bdf cd1372a6 *......$.......r.*
000180 5b598bc3 f7260b00 03f85a58 03c383d2 *[Y........ZX....*
000190 002acb7f afc31200 4f532f32 20212120 *.*......OS/2 !! *
0001a0 53595330 31343735 0d0a0012 004f532f *SYS01475.....OS/*
0001b0 32202121 20535953 30323032 350d0a00 *2 !! SYS02025...*
0001c0 12004f53 2f322021 21205359 53303230 *..OS/2 !! SYS020*
0001d0 32370d0a 004f5332 424f4f54 20202020 *27...OS2BOOT *
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
</font>
</pre>
<h3><a name="T8"><font color="green">
The BPB and other data areas</font></a></h3>
<p>The first 62 bytes of a boot sector are known as the BIOS
Parameter Block (BPB). Here is the layout of the BPB fields
and the values they are assigned in this boot sector:</p>
<pre><font face="Courier New">
db JMP instruction at 7c00 size 2 = eb44
db NOP instruction 7c02 1 90
db OEMname 7c03 8 'IBM 20.0'
dw bytesPerSector 7c0b 2 0200
db sectPerCluster 7c0d 1 01
dw reservedSectors 7c0e 2 0001
db numFAT 7c10 1 02
dw numRootDirEntries 7c11 2 0200
dw numSectors 7c13 2 0000 (use numSectorsHuge)
db mediaType 7c15 1 f8
dw numFATsectors 7c16 2 00d8
dw sectorsPerTrack 7c18 2 003e
dw numHeads 7c1a 2 000e
dd numHiddenSectors 7c1c 4 00000000
dd numSectorsHuge 7c20 4 000d7806
db driveNum 7c24 1 80
db reserved 7c25 1 00
db signature 7c26 1 29
dd volumeID 7c27 4 001c0c23
db volumeLabel 7c2b 11 'NO NAME '
db fileSysType 7c36 8 'FAT '
</font>
</pre>
<p>The first 3 bytes of the BPB are JMP and NOP
instructions.</p>
<pre><font face="Courier New">
0000:7C00 EB44 JMP START
0000:7C02 90 NOP
</font>
</pre>
<p>The rest of the BPB.</p>
<pre><font face="Courier New">
0000:7C00 eb449049 424d2032 302e3000 02100100 *.D.IBM 20.0.....*
0000:7C10 02000200 00f8d800 3e000e00 3e000000 *................*
0000:7C20 06780d00 80002900 1c0c234e 4f204e41 *.x....)...#NO NA*
0000:7C30 4d452020 20204641 54202020 20200000 *ME FAT ..*
</font>
</pre>
<p>Additional data areas.</p>
<pre><font face="Courier New">
0000:7C30 ........ ........ ........ ....0000 * ..*
0000:7C40 00100000 0000.... ........ ........ *...... *
</font>
</pre>
<p>Note:</p>
<ul>
<li>0000:7c3e (DS:003e) = number of sectors in the FATs and root dir.</li>
<li>0000:7c42 (DS:0042) = number of sectors in the FAT.</li>
<li>0000:7c44 (DS:0044) = number of sectors in the root dir.</li>
</ul>
<h3><a name="T10"><font color="green">
Disassembly of the boot sector</font></a></h3>
<pre><font face="Courier New">
START: START OF BOOT SECTOR PROGRAM
0000:7C46 FA CLI interrupts off
0000:7C47 33DB XOR BX,BX zero BX
0000:7C49 8ED3 MOV SS,BX SS now zero
0000:7C4B BCFF7B MOV SP,7BFF SP now 7bff
0000:7C4E FB STI interrupts on
0000:7C4F BAC007 MOV DX,07C0 set DX to
0000:7C52 8EDA MOV DS,DX 07c0
Are we booting from a floppy or a
hard disk partition?
0000:7C54 803E240000 CMP BYTE PTR [0024],00 is driveNum in BPB 00?
0000:7C59 753D JNZ NOT_FLOPPY jmp if not zero
We are booting from a floppy. The
Diskette Parameter Table must be
copied and altered...
Diskette Parameter Table is pointed to by INT 1E. This
program moves this table to high memory, alters the table, and
changes INT 1E to point to the altered table.
This table contains the following data:
????:0000 = Step rate and head unload time.
????:0001 = Head load time and DMA mode flag.
????:0002 = Delay for motor turn off.
????:0003 = Bytes per sector.
????:0004 = Sectors per track.
????:0005 = Intersector gap length.
????:0006 = Data length.
????:0007 = Intersector gap length during format.
????:0008 = Format byte value.
????:0009 = Head settling time.
????:000a = Delay until motor at normal speed.
Compute a valid high memory address.
0000:7C5B 1E PUSH DS save DS
0000:7C5C B84000 MOV AX,0040 set ES
0000:7C5F 8EC0 MOV ES,AX to 0040 (BIOS data area)
0000:7C61 26 ES: reduce system memory
0000:7C62 FF0E1300 DEC WORD PTR [0013] size by 1024
0000:7C66 CD12 INT 12 get system memory size
0000:7C68 C1E06 SHL AX,06 shift AX (mult by 64)
0000:7C6B 8EC0 MOV ES,AX move to ES
0000:7C6D 33FF XOR DI,DI zero DI
Move the diskette param table to high memory.
0000:7C6F 33C0 XOR AX,AX zero AX
0000:7C71 8ED8 MOV DS,AX DS now zero
0000:7C73 C5367800 LDS SI,[0078] DS:SI = INT 1E vector
0000:7C77 FC CLD clear direction
0000:7C78 B90B00 MOV CX,000B count is 11
0000:7C7B F3 REPZ copy diskette param table
0000:7C7C A4 MOVSB to top of memory
Alter the number of sectors per track
in the diskette param table in high memory.
0000:7C7D 1F POP DS restore DS
0000:7C7E A11800 MOV AX,[0018] get sectorsPerTrack from BPB
0000:7C81 26 ES: alter sectors per track
0000:7C82 A20400 MOV [0004],AL in diskette param table
Change INT 1E to point to altered diskette
param table and do a INT 13 disk reset call.
0000:7C85 1E PUSH DS save DS
0000:7C86 33C0 XOR AX,AX AX now zero
0000:7C88 8ED8 MOV DS,AX DS no zero
0000:7C8A A37800 MOV [0078],AX alter INT 1E vector
0000:7C8D 8C067A00 MOV [007A],ES to point to altered
diskette param table
0000:7C91 1F POP DS restore DS
0000:7C92 8A162400 MOV DL,[0024] driveNum from BPB
0000:7C96 CD13 INT 13 diskette reset
NOT_FLOPPY:
Compute the location and the size of
the root directory. Read the entire
root directory into memory.
0000:7C98 A01000 MOV AL,[0010] get numFAT
0000:7C9B 98 CBW make into a word
0000:7C9C F7261600 MUL WORD PTR [0016] mult by numFatSectors
0000:7CA0 03060E00 ADD AX,[000E] add reservedSectors
0000:7CA4 50 PUSH AX save
0000:7CA5 91 XCHG CX,AX move to CX
0000:7CA6 B82000 MOV AX,0020 dir entry size
0000:7CA9 F7261100 MUL WORD PTR [0011] mult by numRootDirEntries
0000:7CAD 8B1E0B00 MOV BX,[000B] get bytesPerSector
0000:7CB1 03C3 ADD AX,BX add
0000:7CB3 48 DEC AX subtract 1
0000:7CB4 F7F3 DIV BX div by bytesPerSector
0000:7CB6 50 PUSH AX save number of dir sectors
0000:7CB7 03C1 ADD AX,CX add number of fat sectors
0000:7CB9 A33E00 MOV [003E],AX save
0000:7CBC B80010 MOV AX,1000 AX is now 1000
0000:7CBF 8EC0 MOV ES,AX ES is now 1000
0000:7CC1 33FF XOR DI,DI DI is now zero
0000:7CC3 59 POP CX get number dir sectors
0000:7CC4 890E4400 MOV [0044],CX save
0000:7CC8 58 POP AX get number fat sectors
0000:7CC9 A34200 MOV [0042],AX save
0000:7CCC 33D2 XOR DX,DX DX now zero
0000:7CCE E87300 CALL READ_SECTOR read 1st sect of root dir
0000:7CD1 33DB XOR BX,BX BX is now zero
0000:7CD3 8B0E1100 MOV CX,[0011] number of root dir entries
DIR_SEARCH: SEARCH FOR OS2BOOT.
Search the root directory for the file
name OS2BOOT.
0000:7CD7 8BFB MOV DI,BX DI is dir entry addr
0000:7CD9 51 PUSH CX save CX
0000:7CDA B90B00 MOV CX,000B count is 11
0000:7CDD BED501 MOV SI,01D5 addr of "OS2BOOT"
0000:7CE0 F3 REPZ is 1st dir entry
0000:7CE1 A6 CMPSB for "OS2BOOT"?
0000:7CE2 59 POP CX restore CX
0000:7CE3 7405 JZ FOUND_OS2BOOT jmp if OS2BOOT
0000:7CE5 83C320 ADD BX,+20 incr to next dir entry
0000:7CE8 E2ED LOOP DIR_SEARCH try again
FOUND_OS2BOOT: FOUND OS2BOOT.
OS2BOOT was found. Get the starting
cluster number and convert to a sector
address. Read OS2BOOT into memory and
finally do a far return to enter
the OS2BOOT program.
0000:7CEA E335 JCXZ FAILED1 JMP if CX zero.
0000:7CEC 26 ES: get the szie of
0000:7CED 8B471C MOV AX,[BX+1C] the OS2BOOT file
0000:7CF0 26 ES: from the OS2BOOT
0000:7CF1 8B571E MOV DX,[BX+1E] directory entry
0000:7CF4 F7360B00 DIV WORD PTR [000B] div by bytesPerSect
0000:7CF8 FEC0 INC AL add 1
0000:7CFA 8AC8 MOV CL,AL num sectors OS2BOOT
0000:7CFC 26 ES: get the starting
0000:7CFD 8B571A MOV DX,[BX+1A] cluster number
0000:7D00 4A DEC DX subtract 1
0000:7D01 4A DEC DX subtract 1
0000:7D02 A00D00 MOV AL,[000D] sectorsPerCluster
0000:7D05 32E4 XOR AH,AH mutiply
0000:7D07 F7E2 MUL DX to get LBA
0000:7D09 03063E00 ADD AX,[003E] add number of FAT sectors
0000:7D0D 83D200 ADC DX,+00 to LBA
0000:7D10 BB0008 MOV BX,0800 set ES
0000:7D13 8EC3 MOV ES,BX to 0800
0000:7D15 33FF XOR DI,DI set ES:DI to entry point
0000:7D17 06 PUSH ES address of
0000:7D18 57 PUSH DI OS2BOOT
0000:7D19 E82800 CALL READ_SECTOR read OS2BOOT into memory
0000:7D1C 8D360B00 LEA SI,[000B] set DS:SI
0000:7D20 CB RETF "far return" to OS2BOOT
FAILED1: OS2BOOT WAS NOT FOUND.
0000:7D21 BE9801 MOV SI,0198 "SYS01475" message
0000:7D24 EB03 JMP FAILED3
FAILED2: ERROR FROM INT 13.
0000:7D26 BEAD01 MOV SI,01AD "SYS02025" message
FAILED3: OUTPUT ERROR MESSAGES.
0000:7D29 E80900 CALL MSG_LOOP display message
0000:7D2C BEC201 MOV SI,01C2 "SYS02027" message
0000:7D2F E80300 CALL MSG_LOOP display message
0000:7D32 FB STI interrupts on
HANG: HANG THE SYSTEM!
0000:7D33 EBFE JMP HANG sit and stay!
MSG_LOOP: DISPLAY AN ERROR MESSAGE.
Routine to display the message
text pointed to by SI.
0000:7D35 AC LODSB get next char of message
0000:7D36 0AC0 OR AL,AL end of message?
0000:7D38 7409 JZ RETURN jmp if yes
0000:7D3A B40E MOV AH,0E write 1 char
0000:7D3C BB0700 MOV BX,0007 video attributes
0000:7D3F CD10 INT 10 INT 10 to write 1 char
0000:7D41 EBF2 JMP MSG_LOOP do again
RETURN:
0000:7D43 C3 RET return
READ_SECTOR: ROUTINE TO READ SECTORS.
Read sectors into memory. Read multiple
sectors but don't read across a track
boundary.
The caller supplies the following:
DX:AX = sector address to read (as LBA)
CX = number of sectors to read
ES:DI = memory address to read into
0000:7D44 50 PUSH AX save lower part of LBA
0000:7D45 52 PUSH DX save upper part of LBA
0000:7D46 51 PUSH CX save number of sect to read
0000:7D47 03061C00 ADD AX,[001C] add numHiddenSectors
0000:7D4B 13161E00 ADC DX,[001E] to LBA
0000:7D4F F7361800 DIV WORD PTR [0018] div by sectorsPerTrack
0000:7D53 FEC2 INC DL add 1 to sector number
0000:7D55 8ADA MOV BL,DL save sector number
0000:7D57 33D2 XOR DX,DX zero upper part of LBA
0000:7D59 F7361A00 DIV WORD PTR [001A] div by numHeads
0000:7D5D 8AFA MOV BH,DL save head number
0000:7D5F 8BD0 MOV DX,AX save cylinder number
0000:7D61 A11800 MOV AX,[0018] sectorsPerTrack
0000:7D64 2AC3 SUB AL,BL sub sector number
0000:7D66 40 INC AX add 1
0000:7D67 50 PUSH AX save number of sector to read
0000:7D68 B402 MOV AH,02 INT 13 read sectors
0000:7D6A B106 MOV CL,06 shift count
0000:7D6C D2E6 SHL DH,CL shift high cyl left
0000:7D6E 0AF3 OR DH,BL or in sector number
0000:7D70 8BCA MOV CX,DX move cyl/sect to CX
0000:7D72 86E9 XCHG CH,CL swap cyl/sect
0000:7D74 8A162400 MOV DL,[0024] driveNum
0000:7D78 8AF7 MOV DH,BH head number
0000:7D7A 8BDF MOV BX,DI memory addr to read into
0000:7D7C CD13 INT 13 INT 13 read sectors call
0000:7D7E 72A6 JB FAILED2 jmp if any error
0000:7D80 5B POP BX get number of sectors read
0000:7D81 59 POP CX restore CX
0000:7D82 8BC3 MOV AX,BX number of sector to AX
0000:7D84 F7260B00 MUL WORD PTR [000B] multiply by sector size
0000:7D88 03F8 ADD DI,AX add to memory address
0000:7D8A 5A POP DX restore upper part of LBA
0000:7D8B 58 POP AX resotre lower part of LBA
0000:7D8C 03C3 ADD AX,BX add number of sector just
0000:7D8E 83D200 ADC DX,+00 read to LBA
0000:7D91 2ACB SUB CL,BL decr requested num of sect
0000:7D93 7FAF JG READ_SECTOR jmp if not zero
0000:7D95 C3 RET return
Data not used.
0000:7D90 ........ ....1200 ........ ........ * .. *
Messages here.
0000:7D90 ........ ........ 4f532f32 20212120 * OS/2 !! *
0000:7Da0 53595330 31343735 0d0a0012 004f532f *SYS01475.....OS/*
0000:7Db0 32202121 20535953 30323032 350d0a00 *2 !! SYS02025...*
0000:7Dc0 12004f53 2f322021 21205359 53303230 *..OS/2 !! SYS020*
0000:7Dd0 32370d0a 00...... ........ ........ *27... *
OS/2 loader file name.
0000:7Dd0 ........ ..4f5332 424f4f54 20202020 * OS2BOOT *
Data not used.
0000:7De0 00000000 00000000 00000000 00000000 *................*
0000:7Df0 00000000 00000000 00000000 0000.... *.............. *
The last two bytes contain a 55AAH signature.
0000:7Df0 ........ ........ ........ ....55aa * U.*
</font>
</pre>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: Master Boot Record</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>Master Boot Record (MBR)</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">Disassembly of a Master Boot Record (MBR)</font></h2>
<p>This article is a disassembly of a Master Boot Record
(MBR). The MBR is the sector at cylinder 0, head 0, sector 1
of a hard disk. An MBR is created by the FDISK program. The
FDISK program of all operating systems must create a
functionally similar MBR. The MBR is first of what could be
many partition sectors, each one containing a four entry
partition table.</p>
<p>At the completion of your system’s Power On Self Test
(POST), INT 19 is called. Usually INT 19 tries to read a boot
sector from the first floppy drive. If a boot sector is found
on the floppy disk, the that boot sector is read into memory
at location 0000:7C00 and INT 19 jumps to memory location
0000:7C00. However, if no boot sector is found on the first
floppy drive, INT 19 tries to read the MBR from the first hard
drive. If an MBR is found it is read into memory at location
0000:7c00 and INT 19 jumps to memory location 0000:7c00. The
small program in the MBR will attempt to locate an active
(bootable) partition in its partition table. If such a
partition is found, the boot sector of that partition is read
into memory at location 0000:7C00 and the MBR program jumps to
memory location 0000:7C00. Each operating system has its own
boot sector format. The small program in the boot sector must
locate the first part of the operating system’s kernel
loader program (or perhaps the kernel itself or perhaps a
"boot manager program") and read that into
memory.</p>
<p>INT 19 is also called when the CTRL-ALT-DEL keys are used.
On most systems, CTRL-ALT-DEL causes an short version of the
POST to be executed before INT 19 is called.</p>
<ul>
<li><a href="#T2"><font color="green">
Where stuff is</font></a></li>
<li><a href="#T3"><font color="green">
Summary of what this thing does</font></a></li>
<li><a href="#T4"><font color="green">
Notes (VERY IMPORTANT)</font></a></li>
<li><a href="#T5"><font color="green">
Entire MBR record in hex and ASCII</font></a></li>
<li><a href="#T6"><font color="green">
Disassembly of the MBR</font></a></li>
</ul>
<h3><a name="T2"><font color="green">
Where stuff is</font></a></h3>
<ul>
<li>The MBR program code starts at offset 0000.</li>
<li>The MBR messages start at offset 008b.</li>
<li>The partition table starts at offset 01be.</li>
<li>The signature is at offset 01fe.</li>
</ul>
<h3><a name="T3"><font color="green">
Summary of what this thing does</font></a></h3>
<p>If an active partition is found, that partition’s boot
record is read into 0000:7c00 and the MBR code jumps to
0000:7c00 with SI pointing to the partition table entry that
describes the partition being booted. The boot record program
uses this data to determine the drive being booted from and
the location of the partition on the disk.</p>
<p>If no active partition table enty is found, ROM BASIC is
entered via INT 18. All other errors cause a system hang, see
label HANG.</p>
<h3><a name="T4"><font color="green">
Notes (VERY IMPORTANT)</font></a></h3>
<ol>
<li>The first byte of an active partition table entry is
80.<br> This byte is loaded into the DL register before INT 13
is called to read the boot sector. When INT 13 is called, DL
is the BIOS device number. Because of this, the boot sector
read by this MBR program can only be read from BIOS device
number 80 (the first hard disk). This is one of the reasons
why it is usually not possible to boot from any other hard
disk. </li>
<li>The MBR program uses the CHS based INT 13H AH=02H call to
read the boot sector of the active partition. The location of
the active partition’s boot sector is in the partition
table entry in CHS format. If the drive is >528MB, this
CHS must be a translated CHS (or L-CHS, see my BIOS TYPES
document). No addresses in LBA form are used (another reason
why LBA doesn’t solve the >528MB problem).</li>
</ol>
<h3><a name="T5"><font color="green">
Entire MBR record in hex and ASCII</font></a></h3>
<pre><font face="Courier New">
OFFSET 0 1 2 3 4 5 6 7 8 9 A B C D E F *0123456789ABCDEF*
000000 fa33c08e d0bc007c 8bf45007 501ffbfc *.3.....|..P.P...*
000010 bf0006b9 0001f2a5 ea1d0600 00bebe07 *................*
000020 b304803c 80740e80 3c00751c 83c610fe *.....t....u.....*
000030 cb75efcd 188b148b 4c028bee 83c610fe *.u......L.......*
000040 cb741a80 3c0074f4 be8b06ac 3c00740b *.t....t.......t.*
000050 56bb0700 b40ecd10 5eebf0eb febf0500 *V.......^.......*
000060 bb007cb8 010257cd 135f730c 33c0cd13 *..|...W.._s.3...*
000070 4f75edbe a306ebd3 bec206bf fe7d813d *Ou...........}.=*
000080 55aa75c7 8bf5ea00 7c000049 6e76616c *U.u.....|..Inval*
000090 69642070 61727469 74696f6e 20746162 *id partition tab*
0000a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000d0 696e6720 73797374 656d0000 00000000 *ing system......*
0000e0 00000000 00000000 00000000 00000000 *................*
0000f0 TO 0001af SAME AS ABOVE
0001b0 00000000 00000000 00000000 00008001 *................*
0001c0 0100060d fef83e00 00000678 0d000000 *...........x....*
0001d0 00000000 00000000 00000000 00000000 *................*
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*
</font>
</pre>
<h3><a name="T6"><font color="green">
Disassembly of the MBR</font></a></h3>
<p>This sector is initially loaded into memory at 0000:7c00
but it immediately relocates itself to 0000:0600.</p>
<pre><font face="Courier New">
BEGIN: NOW AT 0000:7C00, RELOCATE
0000:7C00 FA CLI disable int's
0000:7C01 33C0 XOR AX,AX set stack seg to 0000
0000:7C03 8ED0 MOV SS,AX
0000:7C05 BC007C MOV SP,7C00 set stack ptr to 7c00
0000:7C08 8BF4 MOV SI,SP SI now 7c00
0000:7C0A 50 PUSH AX
0000:7C0B 07 POP ES ES now 0000:7c00
0000:7C0C 50 PUSH AX
0000:7C0D 1F POP DS DS now 0000:7c00
0000:7C0E FB STI allow int's
0000:7C0F FC CLD clear direction
0000:7C10 BF0006 MOV DI,0600 DI now 0600
0000:7C13 B90001 MOV CX,0100 move 256 words (512 bytes)
0000:7C16 F2 REPNZ move MBR from 0000:7c00
0000:7C17 A5 MOVSW to 0000:0600
0000:7C18 EA1D060000 JMP 0000:061D jmp to NEW_LOCATION
NEW_LOCATION: NOW AT 0000:0600
0000:061D BEBE07 MOV SI,07BE point to first table entry
0000:0620 B304 MOV BL,04 there are 4 table entries
SEARCH_LOOP1: SEARCH FOR AN ACTIVE ENTRY
0000:0622 803C80 CMP BYTE PTR [SI],80 is this the active entry?
0000:0625 740E JZ FOUND_ACTIVE yes
0000:0627 803C00 CMP BYTE PTR [SI],00 is this an inactive entry?
0000:062A 751C JNZ NOT_ACTIVE no
0000:062C 83C610 ADD SI,+10 incr table ptr by 16
0000:062F FECB DEC BL decr count
0000:0631 75EF JNZ SEARCH_LOOP1 jmp if not end of table
0000:0633 CD18 INT 18 GO TO ROM BASIC
FOUND_ACTIVE: FOUND THE ACTIVE ENTRY
0000:0635 8B14 MOV DX,[SI] set DH/DL for INT 13 call
0000:0637 8B4C02 MOV CX,[SI+02] set CH/CL for INT 13 call
0000:063A 8BEE MOV BP,SI save table ptr
SEARCH_LOOP2: MAKE SURE ONLY ONE ACTIVE ENTRY
0000:063C 83C610 ADD SI,+10 incr table ptr by 16
0000:063F FECB DEC BL decr count
0000:0641 741A JZ READ_BOOT jmp if end of table
0000:0643 803C00 CMP BYTE PTR [SI],00 is this an inactive entry?
0000:0646 74F4 JZ SEARCH_LOOP2 yes
NOT_ACTIVE: MORE THAN ONE ACTIVE ENTRY FOUND
0000:0648 BE8B06 MOV SI,068B display "Invld prttn tbl"
DISPLAY_MSG: DISPLAY MESSAGE LOOP
0000:064B AC LODSB get char of message
0000:064C 3C00 CMP AL,00 end of message
0000:064E 740B JZ HANG yes
0000:0650 56 PUSH SI save SI
0000:0651 BB0700 MOV BX,0007 screen attributes
0000:0654 B40E MOV AH,0E output 1 char of message
0000:0656 CD10 INT 10 to the display
0000:0658 5E POP SI restore SI
0000:0659 EBF0 JMP DISPLAY_MSG do it again
HANG: HANG THE SYSTEM LOOP
0000:065B EBFE JMP HANG sit and stay!
READ_BOOT: READ ACTIVE PARITION BOOT RECORD
0000:065D BF0500 MOV DI,0005 INT 13 retry count
INT13RTRY: INT 13 RETRY LOOP
0000:0660 BB007C MOV BX,7C00
0000:0663 B80102 MOV AX,0201 read 1 sector
0000:0666 57 PUSH DI save DI
0000:0667 CD13 INT 13 read sector into 0000:7c00
0000:0669 5F POP DI restore DI
0000:066A 730C JNB INT13OK jmp if no INT 13
0000:066C 33C0 XOR AX,AX call INT 13 and
0000:066E CD13 INT 13 do disk reset
0000:0670 4F DEC DI decr DI
0000:0671 75ED JNZ INT13RTRY if not zero, try again
0000:0673 BEA306 MOV SI,06A3 display "Errr ldng systm"
0000:0676 EBD3 JMP DISPLAY_MSG jmp to display loop
INT13OK: INT 13 ERROR
0000:0678 BEC206 MOV SI,06C2 "missing op sys"
0000:067B BFFE7D MOV DI,7DFE point to signature
0000:067E 813D55AA CMP WORD PTR [DI],AA55 is signature correct?
0000:0682 75C7 JNZ DISPLAY_MSG no
0000:0684 8BF5 MOV SI,BP set SI
0000:0686 EA007C0000 JMP 0000:7C00 JUMP TO THE BOOT SECTOR
WITH SI POINTING TO
PART TABLE ENTRY
Messages here.
0000:0680 ........ ........ ......49 6e76616c * Inval*
0000:0690 69642070 61727469 74696f6e 20746162 *id partition tab*
0000:06a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000:06b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000:06c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000:06d0 696e6720 73797374 656d00.. ........ *ing system. *
Data not used.
0000:06d0 ........ ........ ......00 00000000 * .....*
0000:06e0 00000000 00000000 00000000 00000000 *................*
0000:06f0 00000000 00000000 00000000 00000000 *................*
0000:0700 00000000 00000000 00000000 00000000 *................*
0000:0710 00000000 00000000 00000000 00000000 *................*
0000:0720 00000000 00000000 00000000 00000000 *................*
0000:0730 00000000 00000000 00000000 00000000 *................*
0000:0740 00000000 00000000 00000000 00000000 *................*
0000:0750 00000000 00000000 00000000 00000000 *................*
0000:0760 00000000 00000000 00000000 00000000 *................*
0000:0770 00000000 00000000 00000000 00000000 *................*
0000:0780 00000000 00000000 00000000 00000000 *................*
0000:0790 00000000 00000000 00000000 00000000 *................*
0000:07a0 00000000 00000000 00000000 00000000 *................*
0000:07b0 00000000 00000000 00000000 0000.... *............ *
The partition table starts at 0000:07be. Each partition table
entry is 16 bytes. This table defines a single primary partition
which is also an active (bootable) partition.
0000:07b0 ........ ........ ........ ....8001 * ....*
0000:07c0 0100060d fef83e00 00000678 0d000000 *...........x....*
0000:07d0 00000000 00000000 00000000 00000000 *................*
0000:07e0 00000000 00000000 00000000 00000000 *................*
0000:07f0 00000000 00000000 00000000 0000.... *............ *
The last two bytes contain a 55AAH signature.
0000:07f0 ........ ........ ........ ....55aa *..............U.*
</font>
</pre>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
</body>
</html>
<html>
<head>
<title>Hale's HIW: CHS Translation</title>
</head>
<body bgcolor="white"> <FONT FACE="Arial">
<H1 ALIGN="center"><FONT COLOR="red">How It Works:
<br>CHS Translation</FONT></H1>
<p><a href="http://ata-atapi.com/">Go to the ATA-ATAPI.COM Home Page</a></p>
<hr>
<h2><font color="blue">Everything There Is To Know About CHS Translation</font></h2>
<p>Here is the most complete explaination of how CHS
Translation works.</p>
<ul>
<li><a href="#T2"><font color="green">
Introduction (READ THIS!)</font></a></li>
<li><a href="#T3"><font color="green">
Definitions</font></a></li>
<li><a href="#T4"><font color="green">
Background and Assumptions</font></a></li>
<li><a href="#T5"><font color="green">
CHS Translation Algorithms</font></a></li>
<li><a href="#T13"><font color="green">
The BIOS Types</font></a></li>
</ul>
<h2><a name="T2"><font color="blue">Introduction (READ THIS!)</font></a></h2>
<p>This is very technical. Please read carefully. There is
lots of information here that can sound confusing the first
time you read it.</p>
<p>Why is an understanding of how a BIOS works so important?
The basic reason is that the information returned by INT 13H
AH=08H is used by FDISK, it is used in the partition table
entries within a partition record (like the Master Boot
Record) that are created by FDISK, and it is used by the small
boot program that FDISK places into the Master Boot Record.
The information returned by INT 13H AH=08H is in
cylinder/head/sector (CHS) format—it is not in LBA
format. The boot processing done by your computer’s BIOS
(INT 19H and INT 13H) is all CHS based.</p>
<p>Read this so that you are not confused by all the false
information going around that says "LBA solves the
>528MB problem".</p>
<p>Read this so that you understand the possible data
integrity problem that a WD EIDE type BIOS creates. Any BIOS
that has a "LBA mode" in the BIOS setup could be a
WD EIDE BIOS. Be very careful and NEVER chage the "LBA
mode" setting after you have partitioned and installed
your software.</p>
<h2><a name="T3"><font color="blue">Definitions</font></a></h2>
<ul>
<li>528MB - The maximun drive capacity that is supported by 1024 cylinders, 16 heads and 63
sectors (1024x16x63x512). This is the limit for CHS addressing in the original IBM PC/XT
and IBM PC/AT INT 13H BIOS.</li>
<li>8GB - The maximum drive capacity that can be supported by 1024 cylinders, 256 heads and
63 sectors (1024x256x63x512). This is the limit for the BIOS INT 13H AH=0xH calls.</li>
<li>ATA - AT Attachment—The real name of what is widely known as IDE.</li>
<li>CE Cylinder - Customer Engineering cylinder. This is the last cylinder in P-CHS mode.
IBM has always reserved this cylinder for use of disk diagnostic programs. Many BIOS do
not account for it correctly. It is of questionable value these days and probably should
be considered obsolete. However, since there is no industry wide agreement, beware. There
is no CE Cylinder reserved in the L-CHS address. Also beware of diagnostic programs that
don’t realize they are operating in L-CHS mode and think that the last L-CHS cylinder
is the CE Cylinder.</li>
<li>CHS - Cylinder/Head/Sector. This is the traditional way to address sectors on a disk.
There are at least two types of CHS addressing: the CHS that is used at the INT 13H
interface and the CHS that is used at the ATA device interface. In the MFM/RLL/ESDI and
early ATA days the CHS used at the INT 13H interface was the same as the CHS used at the
device interface.</li>
</ul>
<p>Today we have CHS translating BIOS types that can use one
CHS at the INT 13H interface and a different CHS at the device
interface. These two types of CHS will be called the logical
CHS or L-CHS and the physical CHS or P-CHS in this document.
L-CHS is the CHS used at the INT 13H interface and P-CHS is
the CHS used at the device interface.</p>
<p>The L-CHS used at the INT 13 interface allows up to 256
heads, up to 1024 cylinders and up to 63 sectors. This allows
support of up to 8GB drives. This scheme started with either
ESDI or SCSI adapters many years ago.</p>
<p>The P-CHS used at the device interface allows up to 16
heads up to 65535 cylinders, and up to 63 sectors. This
allows access to 2^28 sectors (136GB) on an ATA device. When
a P-CHS is used at the INT 13H interface it is limited to 1024
cylinders, 16 heads and 63 sectors. This is where the old
528MB limit originated.</p>
<p>ATA devices may also support LBA at the device interface.
LBA allows access to approximately 2^28 sectors (137GB) on an
ATA device.</p>
<p>A SCSI host adapter can convert a L-CHS directly to an LBA
used in the SCSI read/write commands. On a PC today, SCSI is
also limited to 8GB when CHS addressing is used at the INT 13H
interface.
<ul>
<li>EDPT - Enhanced fixed Disk Parameter Table—This table returns additional
information for BIOS drive numbers 80H and 81H. The EDPT for BIOS drive 80H is pointed to
by INT 41H. The EDPT for BIOS drive 81H is pointed to by INT 46H. The EDPT is a fixed disk
parameter table with an AxH signature byte. This table format returns two sets of CHS
information. One set is the L-CHS and is probably the same as returned by INT 13H AH=08H.
The other set is the P-CHS used at the drive interface. This type of table allows drives
with >1024 cylinders or drives >528MB to be supported. The translated CHS will have
<=1024 cylinders and (probably) >16 heads. The CHS used at the drive interface will
have >1024 cylinders and <=16 heads. It is unclear how the IBM defined CE cylinder
is accounted for in such a table. Compaq probably gets the credit for the original
definition of this type of table.</li>
<li>FDPT - Fixed Disk Parameter Table - This table returns additional information for BIOS
drive numbers 80H and 81H. The FDPT for BIOS drive 80H is pointed to by INT 41H. The FDPT
for BIOS drive 81H is pointed to by INT 46H. A FDPT does not have a AxH signature byte.
This table format returns a single set of CHS information. The L-CHS information returned
by this table is probably the same as the P-CHS and is also probably the same as the L-CHS
returned by INT 13H AH=08H. However, not all BIOS properly account for the IBM defined CE
cylinder and this can cause a one or two cylinder difference between the number of
cylinders returned in the AH=08H data and the FDPT data. IBM gets the credit for the
original definition of this type of table.</li>
<li>LBA - Logical Block Address. Another way of addressing sectors that uses a simple
numbering scheme starting with zero as the address of the first sector on a device. The
ATA standard requires that cylinder 0, head 0, sector 1 address the same sector as
addressed by LBA 0. LBA addressing can be used at the ATA interface if the ATA device
supports it. LBA addressing is also used at the INT 13H interface by the AH=4xH read/write
calls.</li>
<li>L-CHS—Logical CHS. The CHS used at the INT 13H interface by the AH=0xH calls. See
CHS above.</li>
<li>MBR - Master Boot Record (also known as a partition table) - The sector located at
cylinder 0 head 0 sector 1 (or LBA 0). This sector is created by an "FDISK"
utility program. The MBR may be the only partition table sector or the MBR can be the
first of multiple partition table sectors that form a linked list. A partition table entry
can describe the starting and ending sector addresses of a partition (also known as a
logical volume or a logical drive) in both L-CHS and LBA form. Partition table entries use
the L-CHS returned by INT 13H AH=08H. Older FDISK programs may not compute valid LBA
values.</li>
<li>OS - Operating System.</li>
<li>P-CHS—Physical CHS. The CHS used at the ATA device interface. This CHS is also used
at the INT 13H interface by older BIOS’s that do not support >1024 cylinders or
>528MB. See CHS above.</li>
</ul>
<h2><a name="T4"><font color="blue">Background and Assumptions</font></a></h2>
<p>First, please note that this is written with the OS
implementor in mind and that I am talking about the possible
BIOS types as seen by an OS during its hardware configuration
search.</p>
<p>It is very important that you not be confused by all the
misinformation going around these days. All OS’s that
want to be co-resident with another OS (and that is all of the
PC based OS’s that I know of) MUST use INT 13H to
determine the capacity of a hard disk. And that capacity
information MUST be determined in L-CHS mode. Why is this?
Because: 1) FDISK and the partition tables are really L-CHS
based, and 2) MS/PC DOS uses INT 13H AH=02H and AH=03H to read
and write the disk and these BIOS calls are L-CHS based. The
boot processing done by the BIOS is all L-CHS based. During
the boot processing, all of the disk read accesses are done in
L-CHS mode via INT 13H and this includes loading the first of
the OS’s kernel code or boot manager’s code.</p>
<p>Second, because there can be multiple BIOS types in any one
system, each drive may be under the control of a different
type of BIOS. For example, drive 80H (the first hard drive)
could be controlled by the original system BIOS, drive 81H
(the second drive) could be controlled by a option ROM BIOS
and drive 82H (the third drive) could be controlled by a
software driver. Also, be aware that each drive could be a
different type, for example, drive 80H could be an MFM drive,
drive 81H could be an ATA drive, drive 82H could be a SCSI
drive.</p>
<p>Third, not all OS’s understand or use BIOS drive
numbers greater than 81H. Even if there is INT 13H support
for drives 82H or greater, the OS may not use that
support.</p>
<p>Fourth, the BIOS INT 13H configuration calls are:
<ul>
<li>AH=08H, Get Drive Parameters—This call is restricted to drives up to 528MB without
CHS translation and to drives up to 8GB with CHS translation. For older BIOS with no
support for >1024 cylinders or >528MB, this call returns the same CHS as is used at
the ATA interface (the P-CHS). For newer BIOS’s that do support >1024 cylinders or
>528MB, this call returns a translated CHS (the L-CHS). The CHS returned by this call
is used by FDISK to build partition records.</li>
<li>AH=41H, Get BIOS Extensions Support—This call is used to determine if the
IBM/Microsoft Extensions or if the Phoenix Enhanced INT 13H calls are supported for the
BIOS drive number.</li>
<li>AH=48H, Extended Get Drive Parameters—This call is used to determine the CHS
geometries, LBA information and other data about the BIOS drive number.</li>
<li>the FDPT or EDPT—While not actually a call, but instead a data area, the FDPT or
EDPT can return additional information about a drive.</li>
<li>other tables—The IBM/Microsoft extensions provide a pointer to a drive parameter
table via INT 13H AH=48H. The Phoenix enhancement provides a pointer to a drive parameter
table extension via INT 13H AH=48H. These tables are NOT the same as the FDPT or EDPT.</li>
</ul>
<p>Note: The INT 13H AH=4xH calls duplicate the older AH=0xH
calls but use a different parameter passing structure. This
new structure allows support of drives with up to 2^64 sectors
(really BIG drives). While at the INT 13H interface the
AH=4xH calls are LBA based, these calls do NOT require that
the drive support LBA addressing.</p>
<h2><a name="T5"><font color="blue">CHS Translation Algorithms</font></a></h2>
<p>NOTE: Before you send me email about this, read this
entire section. Thanks!</p>
<p>As you read this, don’t forget that all of the boot
processing done by the system BIOS via INT 19H and INT 13H use
only the INT 13H AH=0xH calls and that all of this processing
is done in CHS mode.</p>
<p>First, lets review all the different ways a BIOS can be
called to perform read/write operations and the conversions
that a BIOS must support.
<ul>
<li>An old BIOS (like BIOS type 1 below) does no CHS translation and does not use LBA. It
only supports the AH=0xH calls:</li>
</ul>
<pre><font face="Courier New">
INT 13H (L-CHS == P-CHS) ATA
AH=0xH --------------------------------> device
(L-CHS) (P-CHS)
</font>
</pre>
<ul>
<li>A newer BIOS may support CHS translation and it may support LBA at the ATA interface:</li>
</ul>
<pre><font face="Courier New">
INT 13H L-CHS ATA
AH=0xH --+--> to --+----------------> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
</font>
</pre>
<ul>
<li>A really new BIOS may also support the AH=4xH in addtion to the older AH\0xH calls. This
BIOS must support all possible combinations of CHS and LBA at both the INT 13H and ATA
interfaces:</li>
</ul>
<pre><font face="Courier New">
INT 13H ATA
AH=4xH --+-----------------------------> device
(LBA) | (LBA)
|
| LBA
+--> to ---------------+
P-CHS |
|
INT 13H L-CHS | ATA
AH=0xH --+--> to --+------------+---> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
</font>
</pre>
<p>You would think there is only one L-CHS to P-CHS
translation algorithm, only one L-CHS to LBA translation
algorithm and only one P-CHS to LBA translation algorithm.
But this is not so. Why? Because there is no document that
standardizes such an algorithm. You can not rely on all
BIOS’s and OS’s to do these translations the same
way.</p>
<p>The following explains what is widely accepted as the
"correct" algorithms.</p>
<p>An ATA disk must implement both CHS and LBA addressing and
must at any given time support only one P-CHS at the device
interface. And, the drive must maintain a strick relationship
between the sector addressing in CHS mode and LBA mode.
Quoting the ATA-2 document:</p>
<p>LBA = ( (cylinder * heads_per_cylinder + heads ) *
sectors_per_track ) + sector - 1</p>
<p>where heads_per_cylinder and sectors_per_track are the
current translation mode values.</p>
<p>This algorithm can also be used by a BIOS or an OS to
convert a L-CHS to an LBA as we’ll see below.</p>
<p>This algorithm can be reversed such that an LBA can be
converted to a CHS:</p>
<pre><font face="Courier New">
cylinder = LBA / (heads_per_cylinder * sectors_per_track)
temp = LBA % (heads_per_cylinder * sectors_per_track)
head = temp / sectors_per_track
sector = temp % sectors_per_track + 1
</font>
</pre>
<p>While most OS’s compute disk addresses in an LBA
scheme, an OS like DOS must convert that LBA to a CHS in order
to call INT 13H.</p>
<p>Technically an INT 13H should follow this process when
converting an L-CHS to a P-CHS:</p>
<p>1) convert the L-CHS to an LBA,</p>
<p>2) convert the LBA to a P-CHS,</p>
<p>If an LBA is required at the ATA interface, then this third
step is needed:</p>
<p>3) convert the P-CHS to an LBA.</p>
<p>All of these conversions are done by normal arithmetic.</p>
<p>However, while this is the technically correct way to do
things, certain short cuts can be taken. It is possible to
convert an L-CHS directly to a P-CHS using bit a bit shifting
algorithm. This combines steps 1 and 2. And, if the ATA
device being used supports LBA, steps 2 and 3 are not needed.
The LBA value produced in step 1 is the same as the LBA value
produced in step 3.</p>
<ul>
<li><a href="#T6"><font color="green">AN EXAMPLE</font></a></li>
<li><a href="#T7"><font color="green">BIT SHIFTING INSTEAD</font></a></li>
<li><a href="#T8"><font color="green">A BIT SHIFTING EXAMPLE</font></a></li>
<li><a href="#T9"><font color="green">SO WHAT IS THE PROBLEM?</font></a></li>
<li><a href="#T10"><font color="green">SO WHY IS THIS A PROBLEM IF IT IS HIDDEN INSIDE THE
BIOS?</font></a></li>
<li><a href="#T11"><font color="green">WHY USE CHS AT ALL?</font></a></li>
<li><a href="#T12"><font color="green">DANGER TO YOUR DATA!</font></a></li>
</ul>
<h3><a name="T6"><font color="green">AN EXAMPLE</font></a></h3>
<p>Lets look at an example. Lets say that the L-CHS is 1000
cylinders 10 heads and 50 sectors, the P-CHS is 2000
cylinders, 5 heads and 50 sectors. Lets say we want to access
the sector at L-CHS 2,4,3.</p>
<ul>
<li>step 1 converts the L-CHS to an LBA,</li>
</ul>
<p>lba = 1202 = ( ( 2 * 10 + 4 ) * 50 ) + 3 - 1</p>
<ul>
<li>step 2 converts the LBA to the P-CHS,</li>
</ul>
<p>cylinder = 4 = ( 1202 / ( 5 * 50 )<br>
temp = 202 = ( 1202 % ( 5 * 50 ) )<br>
head = 4 = ( 202 / 50 )<br>
sector = 3 = ( 202 % 50 ) + 1</p>
<ul>
<li>step 3 converts the P-CHS to an LBA,</li>
</ul>
<p>lba = 1202 = ( ( 4 * 5 + 4 ) * 50 ) + 3 - 1</p>
<p>Most BIOS (or OS) software is not going to do all of this
to convert an address. Most will use some other algorithm.
There are many such algorithms.</p>
<h3><a name="T7"><font color="green">BIT SHIFTING INSTEAD</font></a></h3>
<p>If the L-CHS is produced from the P-CHS by 1) dividing the
P-CHS cylinders by N, and 2) multiplying the P-CHS heads by N,
where N is 2, 4, 8, ..., then this bit shifting algorithm can
be used and N becomes a bit shift value. This is the most
common way to make the P-CHS geometry of a >528MB drive fit
the INT 13H L-CHS rules. Plus this algorithm maintains the
same sector ordering as the more complex algorithm above.
Note the following:</p>
<p>Lcylinder = L-CHS cylinder being accessed<br>
Lhead = L-CHS head being accessed<br>
Lsector = L-CHS sector being accessed</p>
<p>Pcylinder = the P-CHS cylinder being accessed<br>
Phead = the P-CHS head being accessed<br>
Psector = P-CHS sector being accessed</p>
<p>NPH = is the number of heads in the P-CHS<br>
N = 2, 4, 8, ..., the bit shift value</p>
<p>The algorithm, which can be implemented using bit shifting
instead of multiply and divide operations is:</p>
<p>Pcylinder = ( Lcylinder * N ) + ( Lhead / NPH );<br>
Phead = ( Lhead % NPH );<br>
Psector = Lsector;</p>
<h3><a name="T8"><font color="green">A BIT SHIFTING EXAMPLE</font></a></h3>
<p>Lets apply this to our example above (L-CHS = 1000,10,50
and P-CHS = 2000, 5, 50) and access the same sector at at
L-CHS 2,4,3.</p>
<p>Pcylinder = 4 = ( 2 * 2 ) + ( 4 / 5 )<br>
Phead = 4 = ( 4 % 5 )<br>
Psector = 3 = 3</p>
<p>As you can see, this produces the same P-CHS as the more
complex method above.</p>
<h3><a name="T9"><font color="green">SO WHAT IS THE PROBLEM?</font></a></h3>
<p>The basic problem is that there is no requirement that a
CHS translating BIOS followed these rules. There are many
other algorithms that can be implemented to perform a similar
function. Today, there are at least two popular
implementions: the Phoenix implementation (described above)
and the non-Phoenix implementations.</p>
<h3><a name="T10"><font color="green">SO WHY IS THIS A PROBLEM
IF IT IS HIDDEN INSIDE THE BIOS?</font></a></h3>
<p>Because a protected mode OS that does not want to use INT
13H must implement the same CHS translation algorithm. If it
doesn’t, your data gets scrambled.</p>
<h3><a name="T11"><font color="green">WHY USE CHS AT ALL?</font></a></h3>
<p>In the perfect world of tomorrow, maybe only LBA will be
used.</p>
<p>But today we are faced with the following problems:
<ul>
<li>Some drives >528MB don’t implement LBA.</li>
<li>Some drives are optimized for CHS and may have lower performance when given commands in
LBA mode. Don’t forget that LBA is something new for the ATA disk designers who have
worked very hard for many years to optimize CHS address handling. And not all drive
designs require the use of LBA internally.</li>
<li>The L-CHS to LBA conversion is more complex and slower than the bit shifting L-CHS to
P-CHS conversion.</li>
<li>DOS, FDISK and the MBR are still CHS based—they use the CHS returned by INT 13H
AH=08H. Any OS that can be installed on the same disk with DOS must understand CHS
addressing.</li>
<li>The BIOS boot processing and loading of the first OS kernel code is done in CHS
mode—the CHS returned by INT 13H AH=08H is used.</li>
<li>Microsoft has said that their OS’s will not use any disk capacity that can not also
be accessed by INT 13H AH=0xH.</li>
</ul>
<p>These are difficult problems to overcome in today’s
industry environment. The result: chaos.</p>
<h3><a name="T12"><font color="green">DANGER TO YOUR DATA!</font></a></h3>
<p>See the description of BIOS Type 7 below to understand why
a WD EIDE BIOS is so dangerous to your data.</p>
<h2><a name="T13"><font color="blue">The BIOS Types</font></a></h2>
<p>I assume the following:</p>
<ol>
<li>All BIOS INT 13H support has been installed by the time the OS starts its boot
processing. I’m don’t plan to cover what could happen to INT 13H once the OS
starts loading its own device drivers.</li>
<li>Drives supported by INT 13H are numbered sequentially starting with drive number 80H
(80H-FFH are hard drives, 00-7FH are floppy drives).</li>
</ol>
<p>And remember, any time a P-CHS exists it may or may not
account for the CE Cylinder properly.</p>
<p>I have identified the following types of BIOS INT 13H
support as seen by an OS during its boot time hardware
configuration determination:</p>
<ul>
<li><a href="#T14"><font color="green">BIOS Type 1</font></a></li>
<li><a href="#T15"><font color="green">BIOS Type 2</font></a></li>
<li><a href="#T16"><font color="green">BIOS Type 3</font></a></li>
<li><a href="#T17"><font color="green">BIOS Type 4</font></a></li>
<li><a href="#T18"><font color="green">BIOS Type 5</font></a></li>
<li><a href="#T19"><font color="green">BIOS Type 6</font></a></li>
<li><a href="#T20"><font color="green">BIOS Type 7</font></a></li>
<li><a href="#T21"><font color="green">BIOS Type 8</font></a></li>
<li><a href="#T22"><font color="green">BIOS Type 9</font></a></li>
<li><a href="#T23"><font color="green">BIOS Type 10</font></a></li>
</ul>
<h3><a name="T14"><font color="green">BIOS Type 1</font></a></h3>
<p>Origin: Original IBM PC/XT.</p>
<p>BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. There is no CHS translation. INT 13H AH=08H
returns the P-CHS. The FDPT should contain the same
P-CHS.</p>
<p>Description: Supports up to 528MB from a table of drive
descriptions in BIOS ROM. No support for >1024 cylinders
or drives >528MB or LBA.</p>
<p>Support issues: For >1024 cylinders or >528MB
support, either an option ROM with an INT 13H replacement (see
BIOS types 4-7) or- a software driver (see BIOS type 8) must
be added to the system.</p>
<h3><a name="T15"><font color="green">BIOS Type 2</font></a></h3>
<p>Origin: Unknown, but first appeared on systems having BIOS
drive type table entries defining >1024 cylinders. Rumored
to have originated at the request of Novell or SCO.</p>
<p>BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H should return a L-CHS with the
cylinder value limited to 1024. Beware, many BIOS perform a
logical AND on the cylinder value. A correct BIOS will limit
the cylinder value as follows:</p>
<p>cylinder = cylinder > 1024 ? 1024 : cylinder;</p>
<p>An incorrect BIOS will limit the cylinder value as follows
(this implementation turns a 540MB drive into a 12MB
drive!):</p>
<p>cylinder = cylinder & 0x03ff;</p>
<p>The FDPT will return a P-CHS that has the full cylinder
value.</p>
<p>Description: For BIOS drive numbers 80H and 81H, this BIOS
type supports >1024 cylinders or >528MB without using a
translated CHS in the FDPT. INT 13H AH=08H truncates
cylinders to 1024 (beware of buggy implementations). The FDPT
can show >1024 cylinders thereby allowing an OS to support
drives >528MB. May convert the L-CHS or P-CHS directly to
an LBA if the ATA device supports LBA.</p>
<p>Support issues: Actual support of >1024 cylinders is OS
specific—some OS’s may be able to place OS specific
partitions spanning or beyond cylinder 1024. Usually all OS
boot code must be within first 1024 cylinders. The FDISK
program of an OS that supports such partitions uses an OS
specific partition table entry format to identify these
paritions. There does not appear to be a standard (de facto
or otherwise) for this unusual partition table entry.
Apparently one method is to place -1 into the CHS fields and
use the LBA fields to describe the location of the partition.
This DOES NOT require the drive to support LBA addressing.
Using an LBA in the partition table entry is just a trick to
get around the CHS limits in the partition table entry. It is
unclear if such a partition table entry will be ignored by an
OS that does not understand what it is. For an OS that does
not support such partitions, either an option ROM with an INT
13H replacement (see BIOS types 4-7) -or- a software driver
(see BIOS type 8) must be added to the system.</p>
<p>Note: OS/2 can place HPFS partitions and Linux can place
Linux partitions beyond or spanning cylinder 1024. (Anyone
know of other systems that can do the same?)</p>
<h3><a name="T16"><font color="green">BIOS Type 3</font></a></h3>
<p>Origin: Unknown, but first appeared on systems having BIOS
drive type table entires defining >1024 cylinders. Rumored
to have originated at the request of Novell or SCO.</p>
<p>BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H can return an L-CHS with more
than 1024 cylinders.</p>
<p>Description: This BIOS is like type 2 above but it allows
up to 4096 cylinders (12 cylinder bits). It does this in the
INT 13H AH=0xH calls by placing two most significant cylinder
bits (bits 11 and 10) into the upper two bits of the head
number (bits 7 and 6).</p>
<p>Support issues: Identification of such a BIOS is
difficult. As long as the drive(s) supported by this type of
BIOS have <1024 cylinders this BIOS looks like a type 2
BIOS because INT 13H AH=08H should return zero data in bits 7
and 6 of the head information. If INT 13H AH=08H returns non
zero data in bits 7 and 6 of the head information, perhaps it
can be assumed that this is a type 3 BIOS. For more normal
support of >1024 cylinders or >528MB, either an option
ROM with an INT 13H replacement (see BIOS types 4-7) -or- a
software driver (see BIOS type 8) must be added to the
system.</p>
<p>Note: Apparently this BIOS type is no longer produced by
any BIOS vendor.</p>
<h3><a name="T17"><font color="green">BIOS Type 4</font></a></h3>
<p>Origin: Compaq. Probably first appeared in systems with
ESDI drives having >1024 cylinders.</p>
<p>BIOS call support: INT 13H AH=0xH and EDPT for BIOS drives
80H and 81H. If the drive has <1024 cylinders, INT 13H
AH=08H returns the P-CHS and a FDPT is built. If the drive
has >1024 cylinders, INT 13H AH=08H returns an L-CHS and an
EDPT is built.</p>
<p>Description: Looks like a type 2 BIOS when an FDPT is
built. Uses CHS translation when an EDPT is used. May
convert the L-CHS directly to an LBA if the ATA device
supports LBA.</p>
<p>Support issues: This BIOS type may support up to four
drives with a EDPT (or FDPT) for BIOS drive numbers 82H and
83H located in memory following the EDPT (or FDPT) for drive
80H. Different CHS translation algorithms may be used by the
BIOS and an OS.</p>
<h3><a name="T18"><font color="green">BIOS Type 5</font></a></h3>
<p>Origin: The IBM/Microsoft BIOS Extensions document. For
many years this document was marked "confidential"
so it did not get wide spread distribution.</p>
<p>BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for
BIOS drives 80H and 81H. INT 13H AH=08H returns an L-CHS.
INT 13H AH=41H and AH=48H should be used to get P-CHS
configuration. The FDPT/EDPT should not be used. In some
implementations the FDPT/EDPT may not exist.</p>
<p>Description: A BIOS that supports very large drives
(>1024 cylinders, >528MB, actually >8GB), and
supports the INT 13H AH=4xH read/write functions. The AH=4xH
calls use LBA addressing and support drives with up to 2^64
sectors. These calls do NOT require that a drive support LBA
at the drive interface. INT 13H AH=48H describes the L-CHS
used at the INT 13 interface and the P-CHS or LBA used at the
drive interface. This BIOS supports the INT 13 AH=0xH calls
the same as a BIOS type 4.</p>
<p>Support issues: While the INT 13H AH=4xH calls are well
defined, they are not implemented in many systems shipping
today. Currently undefined is how such a BIOS should respond
to INT 13H AH=08H calls for a drive that is >8GB.
Different CHS translation algorithms may be used by the BIOS
and an OS.</p>
<p>Note: Support of LBA at the drive interface may be
automatic or may be under user control via a BIOS setup
option. Use of LBA at the drive interface does not change the
operation of the INT 13 interface.</p>
<h3><a name="T19"><font color="green">BIOS Type 6</font></a></h3>
<p>Origin: The Phoenix Enhanced Disk Drive Specification.</p>
<p>BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for
BIOS drives 80H and 81H. INT 13H AH=08H returns an L-CHS.
INT 13H AH=41H and AH=48H should be used to get P-CHS
configuration. INT 13H AH=48H returns the address of the
Phoenix defined "FDPT Extension" table.</p>
<p>Description: A BIOS that supports very large drives
(>1024 cylinders, >528MB, actually >8GB), and
supports the INT 13H AH=4xH read/write functions. The AH=4xH
calls use LBA addressing and support drives with up to 2^64
sectors. These calls do NOT require that a drive support LBA
at the drive interface. INT 13H AH=48H describes the L-CHS
used at the INT 13 interface and the P-CHS or LBA used at the
drive interface. This BIOS supports the INT 13 AH=0xH calls
the same as a BIOS type 4. The INT 13H AH=48H call returns
additional information such as host adapter addresses, DMA
support, LBA support, etc, in the Phoenix defined "FDPT
Extension" table.</p>
<p>Phoenix says this this BIOS need not support the INT 13H
AH=4xH read/write calls but this BIOS is really an
extension/enhancement of the original IBM/MS BIOS so most
implementations will probably support the full set of INT 13H
AH=4xH calls.</p>
<p>Support issues: Currently undefined is how such a BIOS
should respond to INT 13H AH=08H calls for a drive that is
>8GB. Different CHS translation algorithms may be used by
the BIOS and an OS.</p>
<p>Note: Support of LBA at the drive interface may be
automatic or may be under user control via a BIOS setup
option. Use of LBA at the drive interface does not change the
operation of the INT 13 interface.</p>
<h3><a name="T20"><font color="green">BIOS Type 7</font></a></h3>
<p>Origin: Described in the Western Digital Enhanced IDE
Implementation Guide.</p>
<p>BIOS call support: INT 13H AH=0xH and FDPT or EDPT for
BIOS drives 80H and 81H. An EDPT with a L-CHS of 16 heads and
63 sectors is built when "LBA mode" is enabled. An
FDPT is built when "LBA mode" is disabled.</p>
<p>Description: Supports >1024 cylinders or >528MB
using a EDPT with a translated CHS *** BUT ONLY IF *** the
user requests "LBA mode" in the BIOS setup *** AND
*** the drive supports LBA. As long as "LBA mode"
is enabled, CHS translation is enabled using a L-CHS with
<=1024 cylinders, 16, 32, 64, ..., heads and 63 sectors.
Disk read/write commands are issued in LBA mode at the ATA
interface but other commands are issued in P-CHS mode.
Because the L-CHS is determined by table lookup based on total
drive capacity and not by a multiply/divide of the P-CHS
cylinder and head values, it may not be possible to use the
simple (and faster) bit shifting L-CHS to P-CHS
algorithms.</p>
<p>When "LBA mode" is disabled, this BIOS looks like
a BIOS type 2 with an FDPT. The L-CHS used is taken either
from the BIOS drive type table or from the device’s
Identify Device data. This L-CHS can be very different from
the L-CHS returned when "LBA mode" is enabled.</p>
<p>This BIOS may support FDPT/EDPT for up to four drives in
the same manner as described in BIOS type 4.</p>
<p>The basic problem with this BIOS is that the CHS returned
by INT 13H AH=08H changes because of a change in the "LBA
mode" setting in the BIOS setup. This should not happen.
This use or non-use of LBA at the ATA interface should have no
effect on the CHS returned by INT 13H AH=08H. This is the
only BIOS type know to have this problem.</p>
<p>Support issues: If the user changes the "LBA
mode" setting in BIOS setup, INT 13H AH=08H and the
FDPT/EDPT change which may cause *** DATA CORRUPTION ***. The
user should be warned to not change the "LBA mode"
setting in BIOS setup once the drive has been partitioned and
software installed. Different CHS translation algorithms may
be used by the BIOS and an OS.</p>
<h3><a name="T21"><font color="green">BIOS Type 8</font></a></h3>
<p>Origin: Unknown. Perhaps Ontrack’s Disk Manager was
the first of these software drivers. Another example of such
a driver is Micro House’s EZ Drive.</p>
<p>BIOS call support: Unknown (anyone care to help out
here?). Mostly likely only INT 13H AH=0xH are support.
Probably a FDPT or EDPT exists for drives 80H and 81H.</p>
<p>Description: A software driver that "hides" in
the MBR such that it is loaded into system memory before any
OS boot processing starts. These drivers can have up to three
parts: a part that hides in the MBR, a part that hides in the
remaining sectors of cylinder 0, head 0, and an OS device
driver. The part in the MBR loads the second part of the
driver from cylinder 0 head 0. The second part provides a
replacement for INT 13H that enables CHS translation for at
least the boot drive. Usually the boot drive is defined in
CMOS setup as a type 1 or 2 (5MB or 10MB drive). Once the
second part of the driver is loaded, this definition is
changed to describe the true capacity of the drive and INT 13H
is replaced by the driver’s version of INT 13H that does
CHS translation. In some cases the third part, an OS specific
device driver, must be loaded to enable CHS translation for
devices other than the boot device.</p>
<p>I don’t know the details of how these drivers respond
to INT 13H AH=08H or how they set up drive parameter tables
(anyone care to help out here?). Some of these drivers
convert the L-CHS to an LBA, then they add a small number to
the LBA and finally they convert the LBA to a P-CHS. This in
effect skips over some sectors at the front of the disk.</p>
<p>Support issues: Several identified—Some OS
installation programs will remove or overlay these drivers;
some of these drivers do not perform CHS translation using the
same algorithms used by the other BIOS types; special OS
device drivers may be required in order to use these software
drivers For example, under MS Windows the standard FastDisk
driver (the 32-bit disk access driver) must be replaced by a
driver that understands the Ontrack, Micro House, etc, version
of INT 13H. Different CHS translation algorithms may be used
by the driver and an OS.</p>
<p>The hard disk vendors have been shipping these drivers with
their drives over 528MB during the last year and they have
been ignoring the statements of Microsoft and IBM that these
drivers would not be supported in future OS’s. Now it
appears that both Microsoft and IBM are in a panic trying to
figure out how to support some of these drivers in WinNT,
Win95 and OS/2. It is unclear what the outcome of this will
be at this time.</p>
<p>NOTE: THIS IS NOT A PRODUCT ENDORSEMENT! An alternate
solution for an older ISA system is one of the BIOS
replacement cards. This cards have a BIOS option ROM. AMI
makes such a card called the "Disk Extender". This
card replaces the motherboard’s INT 13H BIOS with a INT
13H BIOS that does some form of CHS translation. Another
solution for older VL-Bus systems is an ATA-2 (EIDE) type host
adapter card that provides a option ROM with an INT 13H
replacement.</p>
<h3><a name="T22"><font color="green">BIOS Type 9</font></a></h3>
<p>Origin: SCSI host adapters.</p>
<p>BIOS call support: Probably INT 13H AH=0xH and FDPT for
BIOS drives 80H and 81H, perhaps INT 13H AH=4xH.</p>
<p>Description: Most SCSI host adapters contain an option ROM
that enables INT 13 support for the attached SCSI hard drives.
It is possible to have more than one SCSI host adapter, each
with its own option ROM. The CHS used at the INT 13H
interface is converted to the LBA that is used in the SCSI
commands. INT 13H AH=08H returns a CHS. This CHS will have
<=1024 cylinders, <=256 heads and <=63 sectors. The
FDPT probably will exist for SCSI drives with BIOS drive
numbers of 80H and 81H and probably indicates the same CHS as
that returned by INT 13H AH=08H. Even though the CHS used at
the INT 13H interface looks like a translated CHS, there is no
need to use a EDPT since there is no CHS-to-CHS translation
used. Other BIOS calls (most likely host adapter specific)
must be used to determine other information about the host
adapter or the drives.</p>
<p>The INT 13H AH=4xH calls can be used to get beyond 8GB but
since there is little support for these calls in today’s
OS’s, there are probably few SCSI host adapters that
support these newer INT 13H calls.</p>
<p>Support issues: Some SCSI host adapters will not install
their option ROM if there are two INT 13H devices previously
installed by another INT 13H BIOS (for example, two
MFM/RLL/ESDI/ATA drives). Other SCSI adapters will install
their option ROM and use BIOS drive numbers greater than 81H.
Some older OS’s don’t understand or use BIOS drive
numbers greater than 81H. SCSI adapters are currently faced
with the >8GB drive problem.</p>
<h3><a name="T23"><font color="green">BIOS Type 10</font></a></h3>
<p>Origin: A european system vendor.</p>
<p>BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H.</p>
<p>Description: This BIOS supports drives >528MB but it
does not support CHS translation. It supports only ATA drives
with LBA capability. INT 13H AH=08H returns an L-CHS. The
L-CHS is converted directly to an LBA. The BIOS sets the ATA
drive to a P-CHS of 16 heads and 63 sectors using the
Initialize Drive Parameters command but it does not use this
P-CHS at the ATA interface.</p>
<p>Support issues: OS/2 will probably work with this BIOS as
long as the drive’s power on default P-CHS mode uses 16
heads and 63 sectors. Because there is no EDPT, OS/2 uses the
ATA Identify Device power on default P-CHS, described in
Identify Device words 1, 3 and 6 as the current P-CHS for the
drive. However, this may not represent the correct P-CHS. A
newer drive will have the its current P-CHS information in
Identify Device words 53-58 but for some reason OS/2 does not
use this information.</p>
<hr>
<p><i>This page was last updated on 05 October 1999.</i></p>
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