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17.3.X 'X' Instructions
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<a name="17-03-X"></a>
Prev: <a href="chp17-w3.htm">17.3.W 'W' Instructions </a>
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17.3.X 'X' Instructions
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<a name="17-03-XCHG"></a>
<h3>XCHG -- Exchange Register/Memory with Register</h3>
Opcode Instruction Clocks Description
90 + r XCHG AX,r16 3 Exchange word register with AX
90 + r XCHG r16,AX 3 Exchange word register with AX
90 + r XCHG EAX,r32 3 Exchange dword register with EAX
90 + r XCHG r32,EAX 3 Exchange dword register with EAX
86 /r XCHG r/m8,r8 3 Exchange byte register with EA byte
86 /r XCHG r8,r/m8 3/5 Exchange byte register with EA byte
87 /r XCHG r/m16,r16 3 Exchange word register with EA word
87 /r XCHG r16,r/m16 3/5 Exchange word register with EA word
87 /r XCHG r/m32,r32 3 Exchange dword register with EA dword
87 /r XCHG r32,r/m32 3/5 Exchange dword register with EA dword
Operation
temp <- DEST
DEST <- SRC
SRC <- temp
<b>Description</b>
XCHG exchanges two operands. The operands can be in either order. If a
memory operand is involved, BUS LOCK is asserted for the duration of the
exchange, regardless of the presence or absence of the LOCK prefix or of the
value of the IOPL.
<b>Flags Affected</b>
None
<b>Protected Mode Exceptions</b>
#GP(0) if either operand is in a nonwritable segment; #GP(0) for an
illegal memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault
<b>Real Address Mode Exceptions</b>
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH
<b>Virtual 8086 Mode Exceptions</b>
Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault
<a name="17-03-XLAT"></a>
<h3>XLAT/XLATB -- Table Look-up Translation</h3>
D7 XLAT m8 5 Set AL to memory byte DS:[(E)BX + unsigned AL]
D7 XLATB 5 Set AL to memory byte DS:[(E)BX + unsigned AL]
Operation
IF AddressSize = 16
THEN
AL <- (BX + ZeroExtend(AL))
ELSE (* AddressSize = 32 *)
AL <- (EBX + ZeroExtend(AL));
FI;
<b>Description</b>
XLAT changes the AL register from the table index to the table entry. AL
should be the unsigned index into a table addressed by DS:BX (for an
address-size attribute of 16 bits) or DS:EBX (for an address-size attribute
of 32 bits).
The operand to XLAT allows for the possibility of a segment override. XLAT
uses the contents of BX even if they differ from the offset of the operand.
The offset of the operand should have been moved intoBX/EBX with a previous
instruction.
The no-operand form, XLATB, can be used if the BX/EBX table will always
reside in the DS segment.
<b>Flags Affected</b>
None
<b>Protected Mode Exceptions</b>
#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault
<b>Real Address Mode Exceptions</b>
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH
<b>Virtual 8086 Mode Exceptions</b>
Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
<a name="17-03-XOR"></a>
<h3>XOR -- Logical Exclusive OR</h3>
Opcode Instruction Clocks Description
34 ib XOR AL,imm8 2 Exclusive-OR immediate byte to AL
35 iw XOR AX,imm16 2 Exclusive-OR immediate word to AX
35 id XOR EAX,imm32 2 Exclusive-OR immediate dword to EAX
80 /6 ib XOR r/m8,imm8 2/7 Exclusive-OR immediate byte to r/m
byte
81 /6 iw XOR r/m16,imm16 2/7 Exclusive-OR immediate word to r/m
word
81 /6 id XOR r/m32,imm32 2/7 Exclusive-OR immediate dword to r/m
dword
83 /6 ib XOR r/m16,imm8 2/7 XOR sign-extended immediate byte
with r/m word
83 /6 ib XOR r/m32,imm8 2/7 XOR sign-extended immediate byte
with r/m dword
30 /r XOR r/m8,r8 2/6 Exclusive-OR byte register to r/m
byte
31 /r XOR r/m16,r16 2/6 Exclusive-OR word register to r/m
word
31 /r XOR r/m32,r32 2/6 Exclusive-OR dword register to r/m
dword
32 /r XOR r8,r/m8 2/7 Exclusive-OR byte register to r/m
byte
33 /r XOR r16,r/m16 2/7 Exclusive-OR word register to r/m
word
33 /r XOR r32,r/m32 2/7 Exclusive-OR dword register to r/m
dword
Operation
DEST <- LeftSRC XOR RightSRC
CF <- 0
OF <- 0
<b>Description</b>
XOR computes the exclusive OR of the two operands. Each bit of the result
is 1 if the corresponding bits of the operands are different; each bit is 0
if the corresponding bits are the same. The answer replaces the first
operand.
<b>Flags Affected</b>
CF = 0, OF = 0; SF, ZF, and PF as described in Appendix C; AF is undefined
<b>Protected Mode Exceptions</b>
#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault
<b>Real Address Mode Exceptions</b>
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH
<b>Virtual 8086 Mode Exceptions</b>
Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault
<hr>
Prev: <a href="chp17-w3.htm">17.3.W 'W' Instructions </a>
Next: <a href="app-a.htm">Appendix A Opcode Map</a>
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