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Why did the 8086 have the LOCK prefix? I think of it as being mostly useful in parallel contexts, e.g. lock cmpxchg for an atomic compare and swap. But the 8086 was a single core, single thread processor, and I'd be surprised if it were used in multiprocessor machines. Oh! did it have anything to do with the 8087?

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In addition to what was said about the viability of SMP, the lock prefix is handy for interrupt safe ops without wholesale disabling interrupts.

It was designed with the intention of being used in SMP configurations too, and I believe some industrial control systems were built that way.

Color me surprised! That's very neat. Thanks for answering the question!

Yes, Intel even had their own bus for this called Multibus. The processor lock signal was connected to the Multibus so that a bus master could lock another boards shared memory for an atomic access.

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