[sponaugle]

Indeed, I suspect most computer engineering students have done some level of CPU design in their coursework. I rather enjoy the design process, and have done many different designs over the years in an attempt to learn about different optimization and design decisions. I typically do something on paper first, then in some simulation, and sometimes into HDL an on an FPGA, or in some cases discrete logic.

I recently did a very simple 16 instruction/16 register RISC-like design (no microcode) built using just 74xx series logic which was successful at over 10MHz, and I then took that design and implemented it in CPLDs to see how it would compress. It really is an enjoyable process and a nice change from the daily software engineering tasks.

Napkin CPU design should be table topic at your next dinner!

[kragen]

That's great! Presumably you mean 74HC or 74HCT or at least 74ALS, not really 74? How much trouble did you get with noise as you pushed it to 10 MHz with SSI chips? How many CPLDs did you end up using?

Thank you for comprehensively rebutting "To be fair, likely none of the readers here have designed a single CPU either :)"

[sponaugle]

Correct! 74HCs for everything, which are both easy to get and forgiving to use. The first build has 4 PCBs ( ALU, Register file, instruction logic, and memory/LEDs/switches). Each PCB is 12-16 74HCx ICs.

I then did a second design of the same thing using Atmel 1504/1508 CPLDs, and that compressed the design to 2 PCBs - One for the CPU itself and a second for system ram, switches, LEDs, etc. That first board had 5 1504s and 1 1508, although it could have been done in a tad less if I used another 1508. The biggest consumer was the register file since it was 16x 8-bit register, which consumes 128 flip flops.

[kragen]

That sounds great! Have you written it up anywhere publicly? It sounds better designed (e.g., using less chips) than most of the SSI-board CPUs I've seen.

I've been thinking it would be fun to see if I could get JLCPCB to build me such a CPU out of SSI with their PCB assembly service. In theory this is nice and simple: I design the processor at the chip/netlist level, debug the thing in a discrete-event or synchronous simulation with Logisim or something, import it into KiCAD, produce some board layouts, send it to them along with US$50 or so, get back a stack of fully populated SMD PCBs, plug them together, plug it in, start single-stepping the clock while watching what's going on, and then ramp the clock up to see how fast it can run reliably. What surprises am I likely to run into?

Unfortunately I don't think Potato Chips' product line covers things like 1GHz 8-bit registers, but if so that would be pretty entertaining.