User's Manual

THE 80286 INSTRUCTION SET
CMPS/CMPSB/CMPSW-Compare string operands
Opcode
A6
A6
A7
Instruction
.CMPS mb,mb
CMPS8
CMPSW
FLAGS MODIFIED
Clocks
8
8
8
Overflow, sign, zero, auxiliary carry, parity, carry
FLAGS UNDEFINED
None
OPERATION
Description
Compare bytes ES:[DI] from [SI]
Compare bytes ES:[DI] from DS:[SI]
Compare words ES:[DI] from DS:[SI]
CMPS compares the byte or word pointed to by SI with the byte or word pointed to by OI by perform-
ing the subtraction
[SI] - [OI]. The result is not placed anywhere; only the flags reflect the result of
the subtraction. The types of the operands
to CMPS determine whether bytes or words are compared.
The segment address ability of the first
(SI) operand determines whether a segment override byte will
be produced or whether the default segment register DS is used. The second (DI) operand must be
addressible from the
ES register; no segment override is possible.
After the comparison
is made, both SI and DI are automatically advanced. If the direction flag is 0
(CLD was executed), the registers increment; if the direction flag is 1 (STD was executed), the regis-
ters decrement. The registers increment or decrement
by 1 if a byte was moved; by 2 if a word was
moved.
CMPS can be preceded by the REPE or REPNE prefix for block comparison 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, DS, or ES segments; #SS(O) for an
illegal address in the
SS segment.
REAL ADDRESS MODE EXCEPTIONS
T ntf"rrl1,.,t 11 fnr ~ wnrn ()np.r~ nn "t ()ff~p.t OFFFFH.
---------r- -- --- -- -----1.
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