Over on a web forum [1] someone asked about some of the older logic chips that required multiple supplies and it's been bothering me that I can't find out the answer.
Some NMOS chips like the 8080 needed an extra negative (-5) and more- positive (+12) supply to function. But pretty soon there were chips like the Z80, which ran from a single supply, but was still described as "NMOS". And Wikipedia says that there was a Soviet clone of the 8080 (the KR580VM80) that used the extra supplies but could (?) operate properly with the -5v supply at 0v and the +12v supply at +5v.
So what were the extra rails for? Textbook illustrations of NMOS gates always seem to show just one supply. What did the internal schematic of the 8080's logic gates look like vs. the Z80's? Why were later NMOS chips able to dispense with the extra rails?
My best guess is that manufacturing allowed closer control of the transistors' Vt or other parameters, and so they could use smaller biasing voltages and still have acceptable margin. But that's really just a guess on my part. Surely someone in this group knows for sure!
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