Z80 mystery solved!
Many moons ago, long before my beard turned gray and my hair fell out, I made extensive use of Z80 microcomputers in systems I designed. I probably built 25 or 30 different systems using that part, so I got to be very
familiar with it. The pinout of the Z80 DIP had one striking oddity: the eight data pins were out-of-order, as if the chip makers had scrambled them. This was in marked contrast to the sixteen address pins, where were in a nice, neat, orderly sequence that wrapped around the top of the package. Why would this be? Why not have all the data pins in order?
Well, now we know
! It took almost 40 years to figure it out...
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