XCHG exchanges the values in its two operands. It can be used
with a LOCK prefix for purposes of multi-processor
synchronisation.
XCHG AX,AX or XCHG EAX,EAX (depending on the BITS
setting) generates the opcode 90h, and so is a synonym for
NOP (NOP).
Every form of xchg the assembler accepts, from NASM's own instruction table.
The second column is the processor that introduced the form; a badge marks a form that needs
an extension, that is lockable, or that long mode dropped.
| Form | Since | Notes |
|---|---|---|
| xchg ax, reg16 | 8086 | |
| xchg rax, reg64 | x86-64 | long |
| xchg reg16, ax | 8086 | |
| xchg reg64, rax | x86-64 | long |
| xchg eax, reg32na | 386 | |
| xchg reg32na, eax | 386 | |
| xchg eax, eax | 386 | nolong |
| xchg reg8, r/m8 | 8086 | lock1 |
| xchg reg16, r/m16 | 8086 | lock1 |
| xchg reg32, r/m32 | 386 | lock1 |
| xchg reg64, r/m64 | x86-64 | lock1long |
| xchg r/m8, reg8 | 8086 | lock |
| xchg r/m16, reg16 | 8086 | lock |
| xchg r/m32, reg32 | 386 | lock |
| xchg r/m64, reg64 | x86-64 | locklong |
