The Censor 932 CPU architecture has one instruction type that takes a 3-bit "index register" field, but looking at the rest of the instruction set, I can't see any instructions that read, write, or otherwise manipulate index registers.

One of the possibilities that occurred to me is that it may just be that index register 1--7 are shadowed onto general register 1--7, but I hesitate to bake that assumption in to my emulator code right now.

Edit: The Censor machine architecture started out as a Standard Radio & Telefon machine in 1967-1968. In 1971 the computer division was split off and merged into Stansaab. In 1973, Facit's terminal divison was merged in. And in 1978, it was merged into Datasaab.

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    Seams like a valid assumption - I go only an instruction table, so hard to tell. On a side note, it would be helpful to add information about a machine/manufacturer in a questions - especially when it's about an unusual beast like the Datasaab 932 – Raffzahn Oct 20 '19 at 12:04
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    agreed, unless you have specific evidence to the contrary, assume that "index register" means "which of the normal registers is being used as an index in this instruction". It is normal in many CPU architectures for regular registers to double as index registers in this way. – Ken Gober Oct 20 '19 at 15:05
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    For now, I am making the assumption that "general register" 1-7 are the same as "index register 1-7". If/when I have more evidence one way or another, I'll either add more comments, or post an answer. – Vatine Dec 21 '19 at 9:48

After having spoken to one of the engineers on the team that built Censor 932, the index registers are R1 to R7, and a "0" index means "do not index".

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    That's pretty standard in most of the CPU architectures I've ever used, apart from maybe old 'the accumulator is a distinct thing and there is only one' architectures like the 7094. – another-dave Apr 30 at 11:08
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    Well, the Z80 have the dedicated IX, IY registers. Which is pretty much why I asked the question in the first place. – Vatine Apr 30 at 12:21

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