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.