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

[Allison]

>
>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:
>> 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.

Actually The article used a CALC chip that was slow on speed but had
an interface that was BCD output and convenient for input.

A few years back I took a 8742 (8048 with a slave interface) 
at 11Mhz and programmed it to do 24bit BCD math (+-/*) and it 
was faster than a calc chip.  It was done because I could.
Overall it was as fast as z80/4mhz.

Allison