FPUs for small computers (Was: What to download for a PDP-8)

Subject: FPUs for small computers (Was: What to download for a PDP-8) From: Jim Battle <frustum at pacbell.net> Date: Wed, 17 Oct 2007 12:09:44 -0500 To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org> woodelf wrote: Northstar (of Northstar Horizon fame) sold an S-100 card that had a TTL FPU on it. It was a microcoded affair (256 words of 40 bits) that processed a nibble (BCD digit) per clock (4 MHz I believe). You could also specify how many digits were in the mantissa (2-14, even # digits only). Only the four basic functions, +-*/, were supported. Northstar had a version of BASIC that could use this FPU. As I recall, you had to specify how many digits of precision you wanted in your floating point numbers when you ordered BASIC (it wasn't dynamically specified in the interpreter). Others sold S-100 cards that used an AMD FPU chip. This too required poking data bytes and a command, then waiting for the result to be computed and then pulling out the result bytes. It wasn't somehow integrated into the instruction set of the host processor. Finally, I recall seeing an article where somebody took a pocket calculator chip and essentially poked simulated keystrokes at it and then decoded the LED driver output to determine the answer. It was very slow, though, so all it saved was the space of the floating point library code.