User's Manual

THE 80286 INSTRUCTION SET
LOCK-Assert BUS LOCK Signal
Opcode
Instruction Clocks
Description
FO LOCK o
Assert BUSLOCK signal for the next instruction
FLAGS MODIFIED
None
FLAGS UNDEFINED
None
OPERATION
LOCK is a prefix that will cause the BUS LOCK signal of the 80286 to be asserted for the duration
of the instruction that it prefixes. In a multiprocessor environment, this signal should
be used to ensure
that the
80286 has exclusive use of any shared memory while BUS LOCK is asserted. The read-
modify-write sequence typically used to implement TEST-AND-SET in the 80286 is the XCHG
instruction.
The
80286 LOCK prefix activates the lock signal for the following instructions: MOYS, INS, and
OUTS. XCHG always asserts BUS LOCK regardless of the presence or absence of the LOCK prefix.
PROTECTED MODE EXCEPTIONS
#GP(O) if the current privilege level is bigger (less privileged) than the .I/O privilege level.
Other exceptions may
be generated by the subsequent (locked) instruction.
REAL ADDRESS MODE EXCEPTIONS
None. Exceptions may still be generated by the subsequent (locked) instruction.
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