3-435
INSTRUCTION SET REFERENCE
MOVS/MOVSB/MOVSW/MOVSDMove Data from StringtoString
Description
These instructions move the byte, word, or doubleword specified with the second operand
(source operand) to the location specified with the first operand (destination operand). Both the
source and destination operands are located in memory. The address of the source operand is
read from the DS:ESI or the DS:SI registers (depending on the address-size attribute of the
instruction, 32 or 16, respectively). The address of the destination operand is read from the
ES:EDI or the ES:DI registers (again depending on the address-size attribute of the instruction).
The DS segment may be overridden with a segment override prefix, but the ES segment cannot
be overridden.
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 MOVS
mnemonic) allows the source and destination operands to be specified explicitly. Here, the
source and destination operands should be symbols that indicate the size and location of the
source value and the destination, respectively. This explicit-operands form is provided to allow
documentation; however, note that the documentation provided by this form can be misleading.
That is, the source and destination operand symbols must specify the correct type (size) of the
operands (bytes, words, or doublewords), but they do not have to specify the correct location.
The locations of the source and destination operands are always specified by the DS:(E)SI and
ES:(E)DI registers, which must be loaded correctly before the move string instruction is
executed.
The no-operands form provides short forms of the byte, word, and doubleword versions of the
MOVS instructions. Here also DS:(E)SI and ES:(E)DI are assumed to be the source and desti-
nation operands, respectively. The size of the source and destination operands is selected with
the mnemonic: MOVSB (byte move), MOVSW (word move), or MOVSD (doubleword move).
After the move operation, the (E)SI and (E)DI registers are incremented or decremented auto-
matically according to the setting of the DF flag in the EFLAGS register. (If the DF flag is 0, the
(E)SI and (E)DI register are incremented; if the DF flag is 1, the (E)SI and (E)DI registers are
decremented.) The registers are incremented or decremented by one for byte operations, by two
for word operations, or by four for doubleword operations.
Opcode
Instruction
Description
A4
MOVS m8, m8
Move byte at address DS:(E)SI to address ES:(E)DI
A5
MOVS m16, m16
Move word at address DS:(E)SI to address ES:(E)DI
A5
MOVS m32, m32
Move doubleword at address DS:(E)SI to address
ES:(E)DI
A4
MOVSB
Move byte at address DS:(E)SI to address ES:(E)DI
A5
MOVSW
Move word at address DS:(E)SI to address ES:(E)DI
A5
MOVSD
Move doubleword at address DS:(E)SI to address
ES:(E)DI
