Intel and compatable CPU's Programming Information
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SCAS/SCASB/SCASW/SCASD -- Compare String Data
Opcode Instruction Clocks Description AE SCAS m8 7 Compare bytes AL-ES:[DI], update (E)DI AF SCAS m16 7 Compare words AX-ES:[DI], update (E)DI AF SCAS m32 7 Compare dwords EAX-ES:[DI], update (E)DI AE SCASB 7 Compare bytes AL-ES:[DI], update (E)DI AF SCASW 7 Compare words AX-ES:[DI], update (E)DI AF SCASD 7 Compare dwords EAX-ES:[DI], update (E)DI
IF AddressSize = 16 THEN use DI for dest-index; ELSE (* AddressSize = 32 *) use EDI for dest-index; FI; IF byte type of instruction THEN AL - [dest-index]; (* Compare byte in AL and dest *) IF DF = 0 THEN IndDec := 1 ELSE IncDec := -1; FI; ELSE IF OperandSize = 16 THEN AX - [dest-index]; (* compare word in AL and dest *) IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI; ELSE (* OperandSize = 32 *) EAX - [dest-index];(* compare dword in EAX & dest *) IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI; FI; FI; dest-index := dest-index + IncDec
DescriptionSCAS subtracts the memory byte or word at the destination register from the AL, AX or EAX register. The result is discarded; only the flags are set. The operand must be addressable from the ES segment; no segment override is possible.
If the address-size attribute for this instruction is 16 bits, DI is used as the destination register; otherwise, the address-size attribute is 32 bits and EDI is used.
The address of the memory data being compared is determined solely by the contents of the destination register, not by the operand to SCAS. The operand validates ES segment addressability and determines the data type. Load the correct index value into DI or EDI before executing SCAS.
After the comparison is made, the destination register is automatically updated. If the direction flag is 0 (CLD was executed), the destination register is incremented; if the direction flag is 1 (STD was executed), it is decremented. The increments or decrements are by 1 if bytes are compared, by 2 if words are compared, or by 4 if doublewords are compared.
SCASB, SCASW, and SCASD are synonyms for the byte, word and doubleword SCAS instructions that don't require operands. They are simpler to code, but provide no type or segment checking.
Flags AffectedOF, SF, ZF, AF, PF, and CF as described in Appendix C
Protected Mode Exceptions#GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault
Real Address Mode ExceptionsInterrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH
Virtual 8086 Mode ExceptionsSame exceptions as in Real Address Mode; #PF(fault-code) for a page fault