User's Manual

THE 80286 INSTRUCTION SET
SCAS/SCASB/SCASW-Compare String Data
Opcode
AE
AF
AE
AF
FLAGS MODIFIED
Instruction
SCAS mb
SCAS mw
SCASB
SCASW
Clocks
7
7
7
7
Overflow, sigri, zero, auxiliary carry, parity, carry
FLAGS UNDEFINED
None
OPERATION
Description
Compare bytes AL - ES:[Olj, advance 01
Compare words AX - ES:[Olj, advance 01
Compare bytes AL - ES:[Olj, advance 01
Compare words AX - ES:[Olj, advance 01
SCAS subtracts the memory byte or word at ES:OI from the AL or AX register. The result is discarded;
only the flags are set. The operand must be addressable from the
ES register; no segment override is
possible.
After the comparison
is made, 01 is automatically advanced. If the direction flag is 0 (CLO was
executed), 01 increments; if the direction flag is I (STO was executed), 01 decrements. DI increments
or decrements by 1 if bytes were compared; by 2 if words were compared.
SCAS can be preceded by the REPE or REPNE prefix for a block search of CX bytes or words. Refer
to the REP instruction for details of this operation.
PROTECTED MODE EXCEPTIONS
#GP(O) for an illegal memory operand effective address in the CS, OS, or ES segments; #SS(O) for an
illegal address
in the SS segment.
REAL ADDRESS MODE EXCEPTIONS
Interrupt 13 for a word operand at offset OFFFFH.
B-100