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 Reversing Sinclair's weird '74 calculator - half ROM of HP35 
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Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1702
Ken Shirriff presents an in-browser emulation, explains the algorithms and includes the commented code (as reversed from photographs of the chip) of this marvellous and weird 1974 calculator from Sinclair. A nibble-serial machine with just 3 registers, no stack, no RAM and an 11-bit opcode format.

See for the emulator, source code and explanation.

The remarkable thing being that the chip was intended to act as a four-function calculator, and indeed patents show an implementation of such a calculator which uses all 320 words(*). To fit a scientific calculator into that ROM, which is half the size of the HP-35's ROM, is amazing - TI said it couldn't be done. Admittedly the result is difficult to use, slow, and inaccurate. Nonetheless I bought two of them, one of which supplied the chip which Ken analysed.



(*) Emulated and explained at

Fri Aug 30, 2013 4:56 pm
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