[adrian_b]

80386 (1985) did not have a branch predictor, which was used first only in Intel Pentium (1993).

Nevertheless, the states of the internal pipelines, which were supposed to be stopped, flushed and restarted cleanly by taken branches, depended on whether the previous branches had been taken or not taken.

[pm215]

Ah, thanks for that correction -- I jumped straight from "depends on the history of conditional branches" to "branch predictor" without stopping to think that that would have been unlikely in the 386.

[adrian_b]

Before having branch predictors, most CPUs that used any kind of instruction pipelining behaved like a modern CPU where all the branches are predicted as not taken.

Thus on an 80386 or 80486 CPU not taken branches behaved like predicted branches on a modern CPU and taken branches behaved as mispredicted branches on a modern CPU.

The 80386 bug described above was probably caused by some kind of incomplete flushing of some pipeline after a taken branch, which leaved it in a state partially invalid, which could be exposed by a specific sequence of the following instructions.