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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: 0) by Anonymous Coward on Thursday October 24 2019, @11:40PM (1 child)

    by Anonymous Coward on Thursday October 24 2019, @11:40PM (#911425)

    The meaning of CPU has changed over the years... so get over it!

    1959 a CPU was the size of refrigerator or bigger. Yes it memory. It had an ALU. It had special registers (memory restricted to a give use)

    When a chip had the ALU and registers. It was called a CPU too. Memory was pushed out and increased in size.

    Now we call those cores in a CPU

    We even have CPU with memory on the. But the term is computer on a chip.

    Hell a Raspinerry is a CPU also.

    It is all a frame of reference and stupid marketing hype now

  • (Score: 0) by Anonymous Coward on Friday October 25 2019, @04:05AM

    by Anonymous Coward on Friday October 25 2019, @04:05AM (#911494)

    This is true. Nearly 30 years ago when I started using computers regularly, the CPU was what you connected the peripherals and monitor to. It contained a processor, RAM, motherboard and some sort of storage medium. Over the last 15-20 years the term CPU has morphed to refer to just the processor and the term computer has morphed, in many people's minds, to be what the CPU was previously.

    It's rather unfortunate, as the changes leave ambiguity that didn't previously exist. But, at this point, there's not much point in arguing as an increasing portion of geeks are generally incompetent on technological issues to the point where this is small potatoes compared with the incompetent UI design and terrible programming languages.