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(nasm.info)Section B.4.32

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B.4.32. `COMISD': Scalar Ordered Double-Precision FP Compare and Set EFLAGS
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     COMISD xmm1,xmm2/mem64        ; 66 0F 2F /r     [WILLAMETTE,SSE2]

   `COMISD' compares the low-order double-precision FP value in the two
source operands. ZF, PF and CF are set according to the result. OF, AF
and AF are cleared. The unordered result is returned if either source
is a NaN (QNaN or SNaN).

   The destination operand is an `XMM' register. The source can be
either an `XMM' register or a memory location.

   The flags are set according to the following rules:

        Result          Flags        Values

        UNORDERED:      ZF,PF,CF <-- 111;
        GREATER_THAN:   ZF,PF,CF <-- 000;
        LESS_THAN:      ZF,PF,CF <-- 001;
        EQUAL:          ZF,PF,CF <-- 100;