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(nasm.info)Section B.4.178

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B.4.178. `MOVSB', `MOVSW', `MOVSD': Move String
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     MOVSB                         ; A4                   [8086]
     MOVSW                         ; o16 A5               [8086]
     MOVSD                         ; o32 A5               [386]

   `MOVSB' copies the byte at `[DS:SI]' or `[DS:ESI]' to `[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) `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 movsb'). The use of `ES' for the store to `[DI]' or `[EDI]' cannot
be overridden.

   `MOVSW' and `MOVSD' work in the same way, but they copy 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.