User's Manual

THE 80286 INSTRUCTION SET
The REP prefixes make sense only in the contexts listed above. They cannot be applied to anything
other than string operations.
Synonymous forms of REPE and REPNE are REPZ and REPNZ, respectively.
The REP prefixes apply only
to one string instruction at a time. To repeat a block of instructions, use
a
LOOP construct.
The precise action for each iteration
is as follows:
1. Check the CX register. If it is zero, exit the iteration and move to the next instruction.
2. Acknowledge any pending interrupts.
3. Perform the string operation once.
4. Decrement CX by 1; no flags are modified.
5. If the string operation is SCAS or CMPS, check the zero flag. If the repeat condition does not
hold, then exit the iteration and
move to the next instruction. Exit if the prefix is REPE and
ZF=O (the last comparison was not equal), or if the prefix is REPNE and ZF=1 (the last
comparison
was equal).
6. Go to step 1 for the next iteration.
As defined by the individual string-ops, the direction of movement through the block is determined by
the direction flag. If the direction flag is 1 (STD was executed), SI and/or DI start at the end of the
block and
move backward; if the direction flag is 0 (CLD was executed), SI and/or DI start at the
beginning of the block and
move forward.
For repeated
SCAS and CMPS operations the repeat can be exited for one of two different reasons:
the CX count can
be exhausted or the zero flag can fail the repeat condition. Your code will probably
want
to distinguish between the two cases. It can do so via either the JCXZ instruction or the condi-
tional jumps that test the zero flag (JZ, JNZ, JE, and JNE).
NOTE
Not all input/output ports can handle the rate at which the repeated I/O instructions execute.
PROTECTED MODE EXCEPTIONS
None by REP; exceptions can be generated when the string-op is executed.
REAL ADDRESS MODE EXCEPTIONS
None by REP; exceptions can be generated when the string-op is executed.
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