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View Full Version : Has anyone found a strange acting NEC 8080 in an early IMSAI



olddataman
May 5th, 2005, 03:44 PM
In the spring of 1976 I sold some IMSAI 8080s that were included in my initial order from them when I became a dealer in early Feb. 1976. One of them went to a man named John Clarke in Shamburg, IL. (who a little later opened The Data Domain of Shamburg. one of the more successful computer stores started in 1876). John was an engineer from Motorola, and a very nice guy and a smart cookie.
He called me one day to tell me that the 8080 chip in his computer was bad. I said that we wold call IMAI and see if there were an more bad ones around and get a replacement. A day or so later he called me again to tell me that the problem was in the design or prodction of the NEC 8080 chip, because he could prove that under certain instruction sequences one instruction failed and cause calculation errors. I don't remember what instruction it was, but he was right and NEC had to redo their maskand replace all of the 8080s made up to that time. So, if you have an early IMSAI and it has a NEC chip you might have a rare one indeed! One of these days I'll look him up again and ask him for details.

Allison
May 5th, 2005, 07:29 PM
In the spring of 1976 I sold some IMSAI 8080s that were included in my initial order from them when I became a dealer in early Feb. 1976. One of them went to a man named John Clarke in Shamburg, IL. (who a little later opened The Data Domain of Shamburg. one of the more successful computer stores started in 1876). John was an engineer from Motorola, and a very nice guy and a smart cookie.
He called me one day to tell me that the 8080 chip in his computer was bad. I said that we wold call IMAI and see if there were an more bad ones around and get a replacement. A day or so later he called me again to tell me that the problem was in the design or prodction of the NEC 8080 chip, because he could prove that under certain instruction sequences one instruction failed and cause calculation errors. I don't remember what instruction it was, but he was right and NEC had to redo their maskand replace all of the 8080s made up to that time. So, if you have an early IMSAI and it has a NEC chip you might have a rare one indeed! One of these days I'll look him up again and ask him for details.

It is indeed true. The first NEC D8080 was built from spec not a mask copy and like many other Intel products to follow the spec lied. NEC
would quickly follow up with the uD8080AF to correct that. From then on
ALL NEC micros that were not mask exchange were tested and certified
exact functional (8085, D780(z80), 8088, 804x, 8051) even V20 that is an improved 8088 with 8080 compatability mode is still fully socket compatable and binary compatable (but a bit faster) then 8088.

I have examples of both NEC 8080s and they are mostly interchangeable except for some oddities around status flags and handeling of the halt/hold
states.

A side note is that the Z80 would revisit the PV flag issue. So some old 8080 code on Z80 too.


Allison