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INSTRUCTION SET SUMMARY
source and destination strings can be located in the same segment. (This latter condition can also
be achieved by loading the DS and ES segment registers with the same segment selector and
allowing the ESI register to default to the DS register.)
The MOVS instruction moves the string element addressed by the ESI register to the location
addressed by the EDI register. The assembler recognizes three short forms of this instruction,
which specify the size of the string to be moved: MOVSB (move byte string), MOVSW (move
word string), and MOVSD (move doubleword string).
The CMPS instruction subtracts the destination string element from the source string element
and updates the status flags (CF, ZF, OF, SF, PF, and AF) in the EFLAGS register according to
the results. Neither string element is written back to memory. The assembler recognizes three
short forms of the CMPS instruction: CMPSB (compare byte strings), CMPSW (compare
word strings), and CMPSD (compare doubleword strings).
The SCAS instruction subtracts the destination string element from the contents of the EAX,
AX, or AL register (depending on operand length) and updates the status flags according to the
results. The string element and register contents are not modified. The following short forms
of the SCAS instruction specifies the operand length: SCASB (scan byte string), SCASW (scan
word string), and SCASD (scan doubleword string).
The LODS instruction loads the source string element identified by the ESI register into the
EAX register (for a doubleword string), the AX register (for a word string), or the AL register
(for a byte string). The short forms for this instruction are LODSB (load byte string), LODSW
(load word string), and LODSD (load doubleword string). This instruction is usually used in a
loop, where other instructions process each element of the string after they are loaded into the
target register.
The STOS instruction stores the source string element from the EAX (doubleword string), AX
(word string), or AL (byte string) register into the memory location identified with the EDI
register. The short forms for this instruction are STOSB (store byte string), STOSW (store
word string), and STOSD (store doubleword string). This instruction is also normally used in a
loop. Here a string is commonly loaded into the register with a LODS instruction, operated
on by other instructions, and then stored again in memory with a STOS instruction.
The I/O instructions (refer to Section 6.11., I/O Instructions) also perform operations on
strings in memory.
6.10.1.Repeating String Operations
The string instructions described in Section 6.10., String Operations perform one iteration of
a string operation. To operate strings longer than a doubleword, the string instructions can be
combined with a repeat prefix (REP) to create a repeating instruction or be placed in a loop.
When used in string instructions, the ESI and EDI registers are automatically incremented or
decremented after each iteration of an instruction to point to the next element (byte, word, or
doubleword) in the string. String operations can thus begin at higher addresses and work toward
lower ones, or they can begin at lower addresses and work toward higher ones. The DF flag in
