SCAS/SCASB/SCASW/SCASD

Scan String

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
REX.W + AF SCASQ A Valid N.E. Compare RAX with quadword at RDI or EDI then set status flags.
AF SCASD A Valid Valid Compare EAX with doubleword at ES:(E)DI or RDI then set status flags.*
AF SCASW A Valid Valid Compare AX with word at ES:(E)DI or RDI then set status flags.*
AE SCASB A Valid Valid Compare AL with byte at ES:(E)DI or RDI then set status flags.*
REX.W + AF SCAS m64 A Valid N.E. Compare RAX with quadword at RDI or EDI then set status flags.
AF SCAS m32 A Valid Valid Compare EAX with doubleword at ES(E)DI or RDI then set status flags.*
AF SCAS m16 A Valid Valid Compare AX with word at ES:(E)DI or RDI, then set status flags.*
AE SCAS m8 A Valid Valid Compare AL with byte at ES:(E)DI or RDI, then set status flags.*

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA NA

Description

In non-64-bit modes and in default 64-bit mode: this instruction compares a byte, word, doubleword or quadword specified using a memory operand with the value in AL, AX, or EAX. It then sets status flags in EFLAGS recording the results. The memory operand address is read from ES:(E)DI register (depending on the address-size attribute of the instruction and the current operational mode). Note that ES cannot be overridden with a segment override prefix.

At the assembly-code level, two forms of this instruction are allowed. The explicit-operand form and the no-operands form. The explicit-operand form (specified using the SCAS mnemonic) allows a memory operand to be specified explicitly. The memory operand must 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 (AL register for byte comparisons, AX for word comparisons, EAX for doubleword comparisons). The explicit-operand form is provided to allow documentation. Note that the documentation 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 ES:(E)DI.

The no-operands form of the instruction uses a short form of SCAS. Again, ES:(E)DI is assumed to be the memory operand and AL, AX, or EAX is assumed to be the register operand. The size of operands is selected by 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 register is incremented or decremented by 1 for byte operations, by 2 for word operations, and by 4 for doubleword operations.

SCAS, SCASB, SCASW, SCASD, and SCASQ can be preceded by the REP prefix for block comparisons of ECX bytes, words, doublewords, or quadwords. Often, however, these instructions will be used in a LOOP construct that takes some action based on the setting of status flags. See "REP/REPE/REPZ /REPNE/REPNZ—Repeat String Operation Prefix" in this chapter for a description of the REP prefix.

In 64-bit mode, the instruction's default address size is 64-bits, 32-bit address size is supported using the prefix 67H. Using a REX prefix in the form of REX.W promotes operation on doubleword operand to 64 bits. The 64-bit no-operand mnemonic is SCASQ. Address of the memory operand is specified in either RDI or EDI, and AL/AX/EAX/RAX may be used as the register operand. After a comparison, the destination register is incremented or decremented by the current operand size (depending on the value of the DF flag). See the summary chart at the beginning of this section for encoding data and limits.

Pseudo Code

(* Non-64-bit Mode: *)
IF (Byte cmparison)
	temp = AL - SRC;
	SetStatusFlags(temp);
	IF DF = 0
		(E)DI = (E)DI + 1;
	ELSE
		(E)DI = (E)DI - 1;
	FI;
ELSE
	IF (* Word comparison *)
		temp = AX - SRC;
		SetStatusFlags(temp);
		IF DF = 0
			(E)DI = (E)DI + 2;
		ELSE
			(E)DI = (E)DI - 2;
		FI;
	FI;
ELSE
	IF (* Doubleword comparison *)
		temp = EAX - SRC;
		SetStatusFlags(temp);
		IF DF = 0
			(E)DI = (E)DI + 4;
		ELSE
			(E)DI = (E)DI - 4;
		FI;
	FI;
FI;
(* 64-bit Mode: *)
IF (Byte cmparison)
	temp = AL - SRC;
	SetStatusFlags(temp);
	IF DF = 0
		(R|E)DI = (R|E)DI + 1;
	ELSE
		(R|E)DI = (R|E)DI - 1;
	FI;
ELSE
	IF (* Word comparison *)
		temp = AX - SRC;
		SetStatusFlags(temp);
		IF DF = 0
			(R|E)DI = (R|E)DI + 2;
		ELSE
			(R|E)DI = (R|E)DI - 2;
		FI;
	FI;
ELSE
	IF (* Doubleword comparison *)
		temp = EAX - SRC;
		SetStatusFlags(temp);
		IF DF = 0
			(R|E)DI = (R|E)DI + 4;
		ELSE
			(R|E)DI = (R|E)DI - 4;
		FI;
	FI;
ELSE
	IF (Quadword comparison using REX.W)
		temp = RAX - SRC;
		SetStatusFlags(temp);
		IF DF = 0
			(R|E)DI = (R|E)DI + 8;
		ELSE
			(R|E)DI = (R|E)DI - 8;
		FI;
	FI;
FI;

Flags Affected

The OF, SF, ZF, AF, PF, and CF flags are set according to the temporary result of the comparison.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#GP(0) If the memory address is in a non-canonical form.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#SS If a memory operand effective address is outside the SS segment limit.
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#GP(0) If a memory operand effective address is outside the limit of the ES segment. If the ES register contains a NULL segment selector. If an illegal memory operand effective address in the ES segment is given.