User's Manual

THE 80286 INSTRUCTION SET
LOOP/LOOPcond-Loop Control with CX Counter
Opcode
Instruction
E2 cb LOOP cb
E1
cb
LOOPE cb
EO
cb
LOOPNE cb
EO
.cb
LOOPNZ cb
E1 cb LOOPZ cb
FLAGS MODIFIED
None
FLAGS UNDEFINED
None
OPERATION
Clocks
8,noj=4
8,noj=4
8,noj=4
·8,noj=4
8,noj=4
Description
DEC CX; jump short if CX;toO
DEC CX; jump short if CX;to 0 and equal (ZF = 1)
DEC CX; jump short if CX;to 0 and not equal
(ZF=O)
DEC CX; jump short if CX;toO and ZF=O
DEC CX; jump short if CX;toO and zero (ZF=1)
LOOP first decrements the ex register without changing any of the flags. Then, conditions are checked
as given in the description above for the form of
LOOP being used. If the conditions are met, then an
intra-segment jump
is made. The destination to LOOP is in the range from 126 (decimal) bytes before
the instruction to
127 bytes beyond the instruction.
The
LOOP instructions are intended to provide iteration control and to combine loop index manage-
ment with conditional branching. To use the
LOOP instruction you load an unsigned iteration count
into
ex, then code the LOOP at the end of a series of instructions to be iterated. The destination of
LOOP is,a label that points to the beginning of the iteration.
PROTECTED MODE EXCEPTIONS
#GP(O) if the offset jumped to is beyond the limits of the current code segment.
REAL ADDRESS MODE EXCEPTIONS
None
8-70