B.4.195. `OUTSB', `OUTSW', `OUTSD': Output String to I/O Port
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OUTSB ; 6E [186]
OUTSW ; o16 6F [186]
OUTSD ; o32 6F [386]
`OUTSB' loads a byte from `[DS:SI]' or `[DS:ESI]' and writes it to
the I/O port specified in `DX'. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set) `SI' or `ESI'.
The register used is `SI' if the address size is 16 bits, and `ESI'
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 outsb').
`OUTSW' and `OUTSD' work in the same way, but they output a word or
a doubleword instead of a byte, and increment or decrement the
addressing registers 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.