Gregory's ServerZooming way in on the 386 shows the transistors for two XOR gates. In red, a shift register for the chip's self-test feature contains XOR gates built from pass transistors. Yellow: prefetch queue control circuitry uses a completely different standard-cell XOR circuit. 2/9
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4 comments
 For the 386, Intel started using standard-cell logic. This is now common but was a big step at the time. The idea is that each gate has a standard circuit in a library so a computer can automatically wire the circuit. Faster than manual layout but the rows waste space. 4/9 @kenshirriff I thought the parts would be much smaller, even in 1985. In this picture it is small, but imaginable. @hambach18 Yes, the functional blocks of the 386 are easily visible to the naked eye, and you can almost imagine seeing individual transistors if you squint really hard :-) |
The XOR (exclusive-or) gate is a logic gate that produces a 1 if one input or the other is a 1, but not both. This is useful for adding, flipping bits, and other functions. XOR is harder to implement than other gates, so chips use a variety of ways to implement it. 3/9