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(nasm.info)Section B.4.286

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B.4.286. `SCASB', `SCASW', `SCASD': Scan String
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     SCASB                         ; AE                   [8086]
     SCASW                         ; o16 AF               [8086]
     SCASD                         ; o32 AF               [386]

   `SCASB' compares the byte in `AL' 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) `DI' (or `EDI').

   The register used is `DI' if the address size is 16 bits, 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.

   Segment override prefixes have no effect for this instruction: the
use of `ES' for the load from `[DI]' or `[EDI]' cannot be overridden.

   `SCASW' and `SCASD' work in the same way, but they compare a word to
`AX' or a doubleword to `EAX' instead of a byte to `AL', 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.