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.