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

[Jim Battle]

woodelf wrote:
> Jules Richardson wrote:
> 
>> For a math copro to be called as such, does it have to integrate 
>> itself with the CPU's instruction set - or does some sort of box of 
>> tricks sitting on an I/O port and accessed through normal CPU 
>> instructions count? I'm not sure what the correct definition is.
> 
> It does tricks, but then with only 8 opcodes defined on the PDP-8
> what do you expect.
> 
>> What do you mean by 'smallest computer' by the way? Smallest in terms 
>> of spec? I'm pretty sure some of the 1950's stuff had optional 
>> floating point hardware available, although it wouldn't have been 
>> physically small :)

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.