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(nasm.info)Section B.4.303

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B.4.303. `STOSB', `STOSW', `STOSD': Store Byte to String
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     STOSB                         ; AA                   [8086]
     STOSW                         ; o16 AB               [8086]
     STOSD                         ; o32 AB               [386]

   `STOSB' stores the byte in `AL' 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 store to `[DI]' or `[EDI]' cannot be overridden.

   `STOSW' and `STOSD' work in the same way, but they store the word in
`AX' or the doubleword in `EAX' instead of the byte in `AL', 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.