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(nasm.info)Section B.4.27

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B.4.27. `CMPSB', `CMPSW', `CMPSD': Compare Strings
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     CMPSB                         ; A6                   [8086]
     CMPSW                         ; o16 A7               [8086]
     CMPSD                         ; o32 A7               [386]

   `CMPSB' compares the byte at `[DS:SI]' or `[DS:ESI]' with the byte
at `[ES:DI]' or `[ES:EDI]', and sets the flags accordingly. It then
increments or decrements (depending on the direction flag: increments
if the flag is clear, decrements if it is set) `SI' and `DI' (or `ESI'
and `EDI').

   The registers used are `SI' and `DI' if the address size is 16 bits,
and `ESI' and `EDI' if it is 32 bits. If you need to use an address
size not equal to the current `BITS' setting, you can use an explicit
`a16' or `a32' prefix.

   The segment register used to load from `[SI]' or `[ESI]' can be
overridden by using a segment register name as a prefix (for example,
`ES CMPSB'). The use of `ES' for the load from `[DI]' or `[EDI]' cannot
be overridden.

   `CMPSW' and `CMPSD' work in the same way, but they compare a word or
a doubleword instead of a byte, and increment or decrement the
addressing registers by 2 or 4 instead of 1.

   The `REPE' and `REPNE' prefixes (equivalently, `REPZ' and `REPNZ')
may be used to repeat the instruction up to `CX' (or `ECX' - again, the
address size chooses which) times until the first unequal or equal byte
is found.