Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 1678 / 1898

4:376 Volume 4: Base IA-32 Instruction Reference
STOS/STOSB/STOSW/STOSD—Store String Data
Description
Stores a byte, word, or doubleword from the AL, AX, or EAX register, respectively, into
the destination operand. The destination operand is a memory location at the address
ES:EDI. (When the operand-size attribute is 16, the DI register is used as the
source-index register.) The ES segment cannot be overridden with a segment override
prefix.
The STOSB, STOSW, and STOSD mnemonics are synonyms of the byte, word, and
doubleword versions of the STOS instructions. They are simpler to use, but provide no
type or segment checking. (For the STOS instruction, “ES:EDI” must be explicitly
specified in the instruction.)
After the byte, word, or doubleword is transfer from the AL, AX, or EAX register to the
memory location, the EDI register is incremented or decremented automatically
according to the setting of the DF flag in the EFLAGS register. (If the DF flag is 0, the
EDI register is incremented; if the DF flag is 1, the EDI register is decremented.) The
EDI register is incremented or decremented by 1 for byte operations, by 2 for word
operations, or by 4 for doubleword operations.
The STOS, STOSB, STOSW, and STOSD instructions can be preceded by the REP prefix
for block loads of ECX bytes, words, or doublewords. More often, however, these
instructions are used within a LOOP construct, because data needs to be moved into the
AL, AX, or EAX register before it can be stored. See
“REP/REPE/REPZ/REPNE /REPNZ—
Repeat String Operation Prefix” on page 4:337 for a description of the REP prefix.
Operation
IF (byte store)
THEN
DEST

AL;
THEN IF DF = 0
THEN (E)DI

1;
ELSE (E)DI

-1;
FI;
ELSE IF (word store)
THEN
DEST

AX;
THEN IF DF = 0
THEN DI

2;
ELSE DI

-2;
FI;
ELSE (* doubleword store *)
Opcode Instruction Description
AA STOS ES:(E)DI Store AL at address ES:(E)DI
AB STOS ES:DI Store AX at address ES:DI
AB STOS ES:EDI Store EAX at address ES:EDI
AA STOSB Store AL at address ES:(E)DI
AB STOSW Store AX at address ES:DI
AB STOSD Store EAX at address ES:EDI