Intel documentation describes the 386's Protection Test Unit as a component that "implements fast testing of complex memory protection functions." It is mostly composed of a PLA (Programmable Logic Array), referred to as Test PLA by Intel, that is physically visible by its regular appearance on the die. This single piece of combinational logic replaces what would otherwise be dozens of multi-cycle conditional branches in the microcode. Instead of testing privilege rules sequentially, the microcode issues a single protection test operation, and the PLA evaluates all applicable rules in parallel, producing a complete decision in one evaluation: continue, fault, or redirect to a gate handler.
Author(s): Stepan Savka, Andriy Serednytski, Dmytro Popovych
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I didn't spot this by myself. A reader first tipped me off to the similarities between the U24 …