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How “special register groups” invaded computer dictionaries for decades #VintageComputing #History @kenshirriff

Ken Shirriff gets out his detective hat and finds the source for computer dictionaries using the term “special register groups” for mainframes – does it or did it ever make sense?

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.

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. 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.).

Computer manufacturers should know their systems don’t have special register groups, but they still used the definition. Special register groups are still being taught to the next generation of students!

“Computer operators working with tape-driven Honeywell 800 mainframe computer.” The operators in this photo from the 1960s are presumably taking advantage of the special register groups unique to the Honeywell 800 and 1800 computers. Photo from National Library of Medicine.

Ken’s conclusion:

For some reason, a 1960 definition of “central processing unit” included “special register groups”, an obscure feature from the Honeywell 800 mainframe. This definition was copied and changed for decades, even though it doesn’t make sense. It appears that once something appears in an authoritative glossary, people will reuse it for decades, and obsolete terms may never die out.

See the entire article here, well worth reading.


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