3-629
INSTRUCTION SET REFERENCE
SCAS/SCASB/SCASW/SCASDScan String
Description
These instructions compare the byte, word, or double word specified with the memory operand
with the value in the AL, AX, or EAX register, and sets the status flags in the EFLAGS register
according to the results. The memory operand address is read from either the ES:EDI or the
ES:DI registers (depending on the address-size attribute of the instruction, 32 or 16, respec-
tively). 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-operand form (specified with the SCAS
mnemonic) allows the memory operand to be specified explicitly. Here, the memory operand
should be a symbol that indicates the size and location of the operand value. The register
operand is then automatically selected to match the size of the memory operand (the AL register
for byte comparisons, AX for word comparisons, and EAX for doubleword comparisons). This
explicit-operand form is provided to allow documentation; however, note that the documenta-
tion provided by this form can be misleading. That is, the memory 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 compare string instruction is executed.
The no-operands form provides short forms of the byte, word, and doubleword versions of the
SCAS instructions. Here also ES:(E)DI is assumed to be the memory operand and the AL, AX,
or EAX register is assumed to be the register operand. The size of the two operands is selected
with the mnemonic: SCASB (byte comparison), SCASW (word comparison), or SCASD
(doubleword comparison).
After the comparison, 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
incremented; if the DF flag is 1, the (E)DI register is decremented.) The (E)DI register is incre-
mented or decremented by one for byte operations, by two for word operations, or by four for
doubleword operations.
The SCAS, SCASB, SCASW, and SCASD instructions can be preceded by the REP prefix for
block comparisons of ECX bytes, words, or doublewords. More often, however, these instruc-
tions will be used in a LOOP construct that takes some action based on the setting of the status
flags before the next comparison is made. Refer to REP/REPE/REPZ/REPNE
/REPNZRepeat String Operation Prefix in this chapter for a description of the REP prefix.
OpcodeInstruction
Description
AE
SCAS m8
Compare AL with byte at ES:(E)DI and set status flags
AF
SCAS m16
Compare AX with word at ES:(E)DI and set status flags
AF
SCAS m32
Compare EAX with doubleword at ES(E)DI and set status flags
AE
SCASB
Compare AL with byte at ES:(E)DI and set status flags
AF
SCASW
Compare AX with word at ES:(E)DI and set status flags
AF
SCASD
Compare EAX with doubleword at ES:(E)DI and set status flags
