15.7 Differences From 80286 Real-Address Mode

The 80286 processor implements the bus lock function differently than the 80386. This fact may or may not be apparent to 8086 programs, depending on how the V86 monitor handles the LOCK prefix. LOCKed instructions are sensitive to IOPL; therefore, software designers can choose to emulate its function. If, however, 8086 programs are allowed to execute LOCK directly, programs that use forms of memory locking specific to the 8086 may not execute properly when transported to a specific application of the 80386.

The LOCK prefix and its corresponding output signal should only be used to prevent other bus masters from interrupting a data movement operation. LOCK may only be used with the following 80386 instructions when they modify memory. An undefined-opcode exception results from using LOCK before any other instruction.

Bit test and change: BTS, BTR, BTC.
Exchange: XCHG.
One-operand arithmetic and logical: INC, DEC, NOT, and NEG.
Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.

A locked instruction is guaranteed to lock only the area of memory defined by the destination operand, but may lock a larger memory area. For example, typical 8086 and 80286 configurations lock the entire physical memory space. With the 80386, the defined area of memory is guaranteed to be locked against access by a processor executing a locked instruction on exactly the same memory area, i.e., an operand with identical starting address and identical length.

up: Chapter 15 -- Virtual 8086 Mode
prev: 15.6 Differences From 8086
next: Chapter 16 -- Mixing 16-Bit and 32 Bit Code