Homebuilt TTL and transistor CPUs

[Tony Duell]

> 
> What qualifies a CPU as "homebuilt"?  Would an FPGA implementation qualify
> as one?  If not, why not?  If FPGA is disqualifed, how about CPLD/GAL/PAL?
> 
> I'm just trying to understand the rules of this thing.

I am not sure there are any 'rules' but I would claim a homebuilt CPU 
implies 

1) You had to do some soldering/wirewrapping

2) You didn't use a chip normally classed as a microprocessor _in the 
conventional way_. I have actually considered using a micro, say a Z80, 
to sequence through a ROM which is, say, 32 bits wide. 8 of those bits 
got to the Z80 data lines, the ROM contains mostly NOPs, with some 
JPs/JRs, and the odd IN instruction to read lines from my homebrew data 
path and control conditional jumps to go to theright routine in the ROM 
.The other 24 btis from the ROM control my data path, of course.

While that would use a microprocessor, I would still class the overall 
circuit as a homebuilt CPU

3) You get to define the instruction set (wether by hardwiring, 
microcode, whatever).

By my definition, FPGAs, PALs, GALs, CPLDs, gate+flip-flops, transistors, 
valves, relays, are all fine.

-tony