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Code Mnemonic 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

Description
Moves 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 over-ridden 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 destination 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 automatically 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 1 for byte operations, by 2 for word operations, or by 4 for doubleword operations.

The MOVS, MOVSB, MOVSW, and MOVSD instructions can be preceded by the REP prefix (see "REP/REPE/REPZ/REPNE/REPNZ—Repeat String Operation Prefix" in this chapter) for block moves of ECX bytes, words, or doublewords.
Operands Bytes Clocks
movsb 1 4 NP
movsw 1 4 NP
movsd 1 4 NP
rep movsb 2 3 + ( count of bytes, words or dwords) NP
rep movsw 2 3 + ( count of bytes, words or dwords) NP
rep movsd 2 3 + ( count of bytes, words or dwords) NP

Flags
ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF unaffected
VM unaffected ZF unaffected
RF unaffected AF unaffected
NT unaffected PF unaffected
IOPL unaffected CF unaffected
OF unaffected