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.