3-668
INSTRUCTION SET REFERENCE
STOS/STOSB/STOSW/STOSDStore String
Description
These instructions store a byte, word, or doubleword from the AL, AX, or EAX register, respec-
tively, into the destination operand. The destination operand is a memory location, the address
of which is read from either the ES:EDI or the ES:DI registers (depending on the address-size
attribute of the instruction, 32 or 16, respectively). The ES segment cannot be overridden with
a segment override prefix.
At the assembly-code level, two forms of this instruction are allowed: the explicit-operands
form and the no-operands form. The explicit-operands form (specified with the STOS
mnemonic) allows the destination operand to be specified explicitly. Here, the destination
operand should be a symbol that indicates the size and location of the destination value. The
source operand is then automatically selected to match the size of the destination operand (the
AL register for byte operands, AX for word operands, and EAX for doubleword operands). This
explicit-operands form is provided to allow documentation; however, note that the documenta-
tion provided by this form can be misleading. That is, the destination operand symbol must
specify the correct type (size) of the operand (byte, word, or doubleword), but it does not have
to specify the correct location. The location is always specified by the ES:(E)DI registers, which
must be loaded correctly before the store string instruction is executed.
The no-operands form provides short forms of the byte, word, and doubleword versions of the
STOS instructions. Here also ES:(E)DI is assumed to be the destination operand and the AL,
AX, or EAX register is assumed to be the source operand. The size of the destination and source
operands is selected with the mnemonic: STOSB (byte read from register AL), STOSW (word
from AX), or STOSD (doubleword from EAX).
After the byte, word, or doubleword is transferred from the AL, AX, or EAX register to the
memory location, the (E)DI 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 (E)DI register is incre-
mented; if the DF flag is 1, the (E)DI register is decremented.) The (E)DI register is incremented
or decremented by one for byte operations, by two for word operations, or by four for double-
word operations.
Opcode
Instruction
Description
AA
STOS m8
Store AL at address ES:(E)DI
AB
STOS m16
Store AX at address ES:(E)DI
AB
STOS m32
Store EAX at address ES:(E)DI
AA
STOSB
Store AL at address ES:(E)DI
AB
STOSW
Store AX at address ES:(E)DI
AB
STOSD
Store EAX at address ES:(E)DI
