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(nasm.info)Section B.4.121

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B.4.121. `INSB', `INSW', `INSD': Input String from I/O Port
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     INSB                          ; 6C                   [186]
     INSW                          ; o16 6D               [186]
     INSD                          ; o32 6D               [386]

   `INSB' inputs a byte from the I/O port specified in `DX' and stores
it at `[ES:DI]' or `[ES:EDI]'. 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.

   `INSW' and `INSD' work in the same way, but they input a word or a
doubleword instead of a byte, and increment or decrement the addressing
register by 2 or 4 instead of 1.

   The `REP' prefix may be used to repeat the instruction `CX' (or
`ECX' - again, the address size chooses which) times.

   See also `OUTSB', `OUTSW' and `OUTSD' (*Note Section B.4.195::).