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posted by Fnord666 on Thursday October 24 2019, @04:27PM   Printer-friendly
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.).


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  • (Score: 2) by DannyB on Thursday October 24 2019, @05:02PM

    by DannyB (5839) Subscriber Badge on Thursday October 24 2019, @05:02PM (#911271) Journal

    Maybe 'storage' in some general sense. But there are various characteristics which put them into broad categories.

    It's all storage right? In some sense yes. But jargon makes communication MUCH easier. That is why various professions have jargon.

    It's all storage, but with different characteristics. Speed. Volatility. Capacity. Cost. Mechanical vs Solid State. Etc.

    Cache exists for a reason. The cpu doesn't directly execute instructions from disk, for a reason. Important permanent information is not kept only in RAM / Memory, for a reason (when you remove electrical power).

    So different forms of 'storage' get different terminology. For a reason.

    Joke from BYTE Magazine:
    Customer asks computer dealer . . .
    Q. What is the difference between Static and Dynamic RAM?
    A. Static memory works, and Dynamic memory doesn't.

    --
    People today are educated enough to repeat what they are taught but not to question what they are taught.
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