OT-ish: Build-it stuff

Don Y dgy at DakotaCom.Net
Wed Jun 14 22:04:32 CDT 2006

Tony Duell wrote:
> [ Homebrew procrssors]
>> I think once the 2901 "fell from grace" (?), this became a thing
>> of the past.  I've designed two processors "from scratch" (TTL
> Maybe, althoguh i never cared much for the 2901...

2901 really only makes sense for a "classic" CPU design.
By the time you put all of the parts together to get a complete
CPU, it is quite a large solution.  Some parts (29116, etc.)
could help a little but it still was big.

If you don't need a "full" instruction set (traditional ALU, etc.)
then there were better ways of doing this.  E.g., in one of my
CPUs, I needed some cascaded multiply/inverse/multiply operators.
I chose to use serial adders and multipliers instead of a more
conventional approach.

>> with bipolar ROMs for the microcode store) and found it quite
>> an interesting exercise.  Not just the "logic design" but
>> actually thinking about what the instruction set should be
>> for that particular application domain, etc.
>> But, nowadays, I think it would be a lot less tedious if you
>> could do it in a big FPGA using synthesis tools.  You could
> Hmm... I had to use FPGAs in my last job, and I hated every darn minute 
> of it. Don't get me wrong, I can and will use them if somebody is paying 
> me to do so, but I won't chose them for my own design. I found it a lot 
> quicker to debug a circuit by changing things on the actual hardware 
> (rathen than waiting for your design to complie again, and finding the 
> darn compiler had removed most of your logic without warning because 
> you'd tiend an enable pin to the wrong state). And I wouldn't trust that 
> simulator as far as I could throw it ... No, for my own hobby designs, 
> I'll stick to boards of TTL and a logic analyser.

I think it depends on how big the item that you are designing
will end up -- and, if it stays a "one of a kind" design or if
you ever decide to make several of them.

What I find most annoying about FPGAs is they are all sole
source parts -- if you have a problem with one vendor's
part, you may have to reinvest a lot of time to switch to
another vendor's device (as well as investing in another
tool chain!)

>> So, is it your opinion that the "build it" mentality is
>> so individualistic that the differences in attitudes towards
>> it between our sides of the pond are more *cultural* (of
> I am not sure it is different in the UK. Very few people do any kind of 
> homebrewing over here any more. There are a few of us left, I guess, 
> that's all. 


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