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from the an-organization-of-very-special-registers dept.
http://www.righto.com/2019/10/how-special-register-groups-invaded.html
Half a century ago, the puzzling phrase "special register groups" started showing up in definitions of "CPU", and it is still there. In this blog post, I uncover how special register groups went from an obscure feature in the Honeywell 800 mainframe to appearing in the Washington Post.
While researching old computers, I found a strange definition of "Central Processing Unit" that keeps appearing in different sources. From a book reprinted in 2017:1
"Central Processor Unit (CPU)—Part of a computer system which contains the main storage, arithmetic unit and special register groups. It performs arithmetic operations, controls instruction processing and provides timing signals."
At first glance, this definition seems okay, but a few moments thought reveals some problems. Storage is not part of the CPU. But more puzzling, what are special register groups? A CPU has registers, but "special register groups" is not a normal phrase.
It turns out that this definition has been used extensively for over half a century, even though it doesn't make sense, copied and modified from one source to another. Special register groups were a feature in the Honeywell 800 mainframe computer, introduced in 1959. Although this computer is long-forgotten, its impact inexplicably remains in many glossaries. The Honeywell 800 allowed eight programs to run on a single processor, switching between programs after every instruction.3 To support this, each program had a "special register group" in hardware, its own separate group of 32 registers (program counter, general-purpose registers, index registers, etc.).
(Score: 1, Informative) by Anonymous Coward on Thursday October 24 2019, @05:50PM (4 children)
It did require switching. EX AF, AF' switched AF with its complement, EXX switched BC, DE and HL with theirs. IX and IY had no complements, and neither did the "special register groups".
(Score: -1, Flamebait) by Anonymous Coward on Thursday October 24 2019, @09:20PM
Fuck you.
(Score: 0) by Anonymous Coward on Thursday October 24 2019, @09:31PM (2 children)
My man, give me fist bump.
Zilog inc was like AMD against Intel back then. And MS actually produced some quality shit (like "softcard" for apple ii) back then.
\
(Score: 0) by Anonymous Coward on Friday October 25 2019, @02:39AM (1 child)
👊
Back in the day, all the interesting machines around me having Z80s, which was an improvement on the Intel 8080. M68K was going to power the next generation. I was wondering when Intel was going to disappear.
Alas, Intel is still here and Motorola and Zilog really aren't.
(Score: 2) by hendrikboom on Friday October 25 2019, @07:54PM
Always thought IBM chose the wrong processor for their PC.