Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

7-26 Vol. 1
PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS
ating the ESI register with the ES segment register, both the 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 modi-
fied. The following “short forms” of the SCAS instruction specify 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 iden-
tified 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 (see Section 7.3.11, “I/O Instructions”) also perform operations
on strings in memory.
7.3.9.1 Repeating String Operations
The string instructions described in Section 7.3.9, “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 incre-
mented 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