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XCHG Exchange Operands
Flags affected:
O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry
F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag
<none> IF: Interrupt flag ZF: Zero flag CF: Carry flag
Legal forms:
XCHG r8,r8
XCHG r8,m8
XCHG r16,r16
XCHG r16,m16
XCHG r32,r32 386+
XCHG r32,m32 386+
Examples:
XCHG AL,DH
XCHG BH,BYTE [SI]
XCHG SP,BP
XCHG DX,WORD [DI]
XCHG ESI,EDI
XCHG ECX,DWORD [EBP+38]
XCHG AX,BX ; Uses single-byte opcode
Notes:
XCHG exchanges the contents of its two operands. This is why there is no form of XCHG for identical operands; that is, XCHG AX,AX is not a legal form since exchanging a register with itself makes no logical sense.Exchanging an operand with AX may be accomplished with a single-byte opcode, saving fetch time and code space. All good assemblers recognize these cases and optimize for them, but if you are hand-assembling INLINE statements for some high-level language, keep the single-byte special cases in mind.
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI
sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI
m8 = 8-bit memory data m16 = 16-bit memory data
m32 = 32-bit memory data i8 = 8-bit immediate data
i16 = 16-bit immediate data i32 = 32-bit immediate data
d8 = 8-bit signed displacement d16 = 16-bit signed displacement
d32 = 32-bit unsigned displacement
