Two interesting XOR circuits inside the Intel 386 processor #VintageComputing #Intel #x86 @kenshirriff
Ken Shirriff is reverse engineering the Intel 80386 (‘386) microprocessor.
Intel’s 386 processor (1985) was an important advance in the x86 architecture, not only moving to a 32-bit processor but also switching to a CMOS implementation. I’ve been reverse-engineering parts of the 386 chip and came across two interesting and completely different circuits that the 386 uses to implement an XOR gate: one uses standard-cell logic while the other uses pass-transistor logic. In this article, I take a look at those circuits.
An XOR gate may seem like a trivial circuit, but there is more going on than you might expect. I think it is interesting that there isn’t a single solution for implementing XOR; even inside a single chip, multiple approaches can be used. (If you’re interested in XOR circuits, I also looked at the XOR circuit in the Z80.) It’s also reassuring to see that even for a complex chip such as the 386, the circuitry can be broken down into logic gates and then understood at the transistor level.
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