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

[Allison]

>
>Subject: Re: FPUs for small computers (Was: What to download for a PDP-8)
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Wed, 17 Oct 2007 10:21:40 -0700
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>On 17 Oct 2007 at 12:09, Jim Battle wrote:
>
>> 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.
>
>I remember that one.  It may have been an early issue of Byte.  I 
>also wondered at the time if any speed was gained over just running 
>plain old x80 code.

If it's the article as I remember it it was a push for Add/subtract
but better for multiply and divide (to 8 digits). An 8080 does a 
16x16 multiply in a few milliseconds to compare.

In another Byte there was a simple design and explanation of 
hardware multiply by shift and add.  That was capable of high speed.
With TTL of the time that was something under 10 microseconds 
for 16 bits using a 2mhz clock.  At that point a 4mhz Z80 takes 
nearly that much time to load and read it.   You can do subtract 
and shift to implement a divide. 


Allison