Looking over the 9440 spec sheet, a question arises. The very first line notes that it has eight registers... eight? There's the four AC's, PC, carry bit (all 16-bits for that one!) and... what? On the NOVA3 you would have SP and SO, but this didn't have a stack. Anyone know?

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    Perhaps an Instruction Register? – Brian H Dec 13 '19 at 20:25

Looking over the data sheet referenced, there is also an Instruction Register, and a "Bus Register". The IR is obvious. The BR seems to perform a similar function for fetched data (not instructions).

Together with the 4 accumulators, PC, and SR makes 8 16-bit registers.

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Try reading as far as Pages 2 and 3 of your data sheet:

Four general purpose registers, program counter, and three internal special-purpose registers.

Calling something which "holds the data being transferred from the ALU to the destination accumulator register for one clock cycle" a "scratch register" might have been done by the marketing department who wanted more registers, not the chip designers, but who knows.

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