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LOCK
LOCK THE
BUS
LOCK
Operation:
None
Description:
LOCK
is
a one-byte prefix
that
causes the 8088
(configured in maximum mode) to assert its
bus
LOCK signal while the following instruc-
tion executes.
LOCK does not affect any flags.
The instruction most useful in this context
is
an exchange register with memory. A simple
software lock may be implemented with the
following code sequence:
Encoding:
11110000
Flags Affected:
None
Check:
MOV
AL,1
;set
AL
to
1 (implies locked)
LOCK
XCHG
Sema,AL ;test and set lock
TEST
AL,AL ;set flags based on
AL
JNZ Check
;retry if
lock already set
MOV
Sema,O
;clear the lock when done
The LOCK prefix may be combined with the
segment override
and/or
REP prefixes.
LOCK
Operands
Clocks
Transfers
Bytes
LOCK
Coding
Example
(no operands)
2
- 1
LOCK
XCHG
FLAG,AL
2-111