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(nasm.info)Section B.4.141

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B.4.141. `LODSB', `LODSW', `LODSD': Load from String
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     LODSB                         ; AC                   [8086]
     LODSW                         ; o16 AD               [8086]
     LODSD                         ; o32 AD               [386]

   `LODSB' loads a byte from `[DS:SI]' or `[DS:ESI]' into `AL'. 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 LODSB').

   `LODSW' and `LODSD' work in the same way, but they load a word or a
doubleword instead of a byte, and increment or decrement the addressing
registers by 2 or 4 instead of 1.