Intel 210200-002 manual Lock Lock the BUS Lock

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