CMPXCHG         Compare and Exchange                 Flags: O D I T S Z A P C
                                                            * - - - * * * * *
CMPXCHG destination,source                           CPU: 486+ (*)

        Logic   CMP destination,accumulator
                if ZF = 1
                   destination ← source
                else
                   accumulator ← destination
                endif

    CMPXCHG compares the accumulator (AL, AX, or EAX) with a
    destination operand. If they are equal, the source operand is
    loaded into destination; otherwise, the destination operand is
    loaded into the accumulator.

    This instruction is used to support semaphores.


    Note
    On the A-step of the 486, CMPXCHG was microcoded using the opcodes
    for the, discarded, A- to B0-step 386 instructions XBTS (A6) and
    IBTS (A7). Because of software conflicts with software written for
    the early 386 DX, the opcodes for the 486 were changed to those
    shown below starting with the B step.
    Note that some 386 software won't run on older 386s and some 486
    software will not run on early 486s when using this instruction.


    Opcode      Format
    0F A6 /r    CMPXCHG r/m8,r8         ; i486 pre-B step only
    0F A7 /r    CMPXCHG r/m16,r16       ; i486 pre-B step only
    0F A7 /r    CMPXCHG r/m32,r32       ; i486 pre-B step only
    0F B0 /r    CMPXCHG r/m8,r8
    0F B1 /r    CMPXCHG r/m16,r16
    0F B1 /r    CMPXCHG r/m32,r32


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    reg, reg      3                                      6       5   NP
    mem, reg    3+d(0-2)                                7-10     6   NP