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(nasm.info)Section B.4.333

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B.4.333. `XCHG': Exchange
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     XCHG reg8,r/m8                ; 86 /r                [8086]
     XCHG reg16,r/m8               ; o16 87 /r            [8086]
     XCHG reg32,r/m32              ; o32 87 /r            [386]

     XCHG r/m8,reg8                ; 86 /r                [8086]
     XCHG r/m16,reg16              ; o16 87 /r            [8086]
     XCHG r/m32,reg32              ; o32 87 /r            [386]

     XCHG AX,reg16                 ; o16 90+r             [8086]
     XCHG EAX,reg32                ; o32 90+r             [386]
     XCHG reg16,AX                 ; o16 90+r             [8086]
     XCHG reg32,EAX                ; o32 90+r             [386]

   `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' (*Note
Section B.4.190::).