TTL homebrew CPUs

[Allison]

>
>Subject: Re: TTL homebrew CPUs
>   From: woodelf <bfranchuk at jetnet.ab.ca>
>   Date: Sun, 08 Jul 2007 14:12:15 -0600
>     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>
>Allison wrote:
>> 
>> 
>> There is a lot of "you can do" but building a CPU comes down to what do 
>> you want to do and can reasonably achieve or can practically play with
>> once functional.  Unfortunately some want a cpu that can run unix and an
>> IP stack others want just enough to qualify as a computer for close up 
>> examination of "just how do they do it?" and that represents a large 
>> spread of design and architecture possibilities.  Having built machine 
>> from very simple state logic to an elaborate 16bit bitslice I can say 
>> all are interesting but some are a lot too much work.
>
>Memory is  a interesting problem , if you don't use 8 bit bytes
>or you need more than 32K. Since I had use of computers after
>1980  64Kb or more seems normal. I am shocked
>at how hard it is get any memory size at all using static
>memory with the 75 to 80's chips and still not use
>up mammonth amounts of power or large # or boards.

Why the surprize?  Until the 4k dynamic parts 1k was huge
and the older dynamic parts require a lot of support 
(read TTL bits).  That lack of density says power is big 
by shear force of numbers.  I had the use of computers before
the 70s (ok only 1969) but I got to see the growth.  It was 
interesting to see 4K of core shrink from a box to a board
and further to a chip.

Myself I used in the past the higest density parts memory 
wise I could to save power and wiring. The 2102, 2114 2167 
type static parts made life far easier than the 4116 dynamics.
But by 1978 it was possible to put 64kx8 on a S100 card cheaply
(by then standard) using 16kx1(4116) and 8202.

If I were to do a TTL design right now I'd use 32Kx8 
(or larger) parts even if the word size were 9 or 11 bits 
as they are cheap and easy to use and the unused excess bits 
are no real loss.  I'm  currently looking at using 64Kx8s as 
a way to make a RS08like disk for my PDP-8f.  So what if I 
don't use 4 of the 16bits the cost to find X4 parts or other 
schemes end up using more parts or time.

Maybe because every time I did TTL or slice design I wasn't 
trying to make a period machine or be faithful I was doing 
some sacrelige but I was having fun and the expereince was 
no less because the memory, logic used or terminal was 
way out of period.

Heck the guy that did the Apollo AGC has a hats off to me as
he did in TTL something conceived as RTL with minimal data.
If anything not only was a working machine a significant
accomplishment but the information about it he dug up, made 
visible to public and preserved along the way speaks to 
great work.


For those that try and or succeed to build a PISC 
(Pitiful Instruction Set Computer) or a VSC (Very 
Simple Computer) they are contributing a lot to the 
science and history of computing.  After all there 
are many old (really old) machine preserved and sitting
that most of us have not the first idea how to power 
up and program.  Those that do it get my attention for 
their efforts.


  Allison

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