ThawnyRose

CPU project - history

Somefur recently made the mistake of asking me to talk about my CPU project - here's how that went!

The original question was was "Did you know of any resources, or were you just powering through making it entirely from a base level understanding of the logic?"

I responded with:

Okay, this is actually a funny story (at least I think so!) I started the project in high school with an online circuit simulator, and actually used the minecraft redstone computer guide on the old minecraft wiki for a 4 bit computer! While I later learned that this was a risc cpu, I switched to 8 bit and naively powered on trying to implement complex instructions in a single clock cycle x3 (Ahh, cluelessness...)

At some point I realized this was impossible, but that every step started with the same three steps: increment the PC, load the instruction, and execute it. Somehow I had the idea (maybe seeing other stuff online? I can't remember now) to generalize all the things the cpu could do into what I called the microarchitecture (microcode) that was based on moving to and from: 0b0000-0001 would have enabled the output enable on register 00 and the input on register 01, etc... (with exceptions for arthithmatic)

The final version of the logic level simulator would load an instruction, and use that as an index into the microarchitecture ROM, effectively running a nested risc cpu inside to execute the microarchtecture! An immediate load instruction might look like:

  1. (Hardcoded) Inc PC
  2. (Hardcoded) MAR -> instruction register
  3. (Hardcoded) Latch microcode/microprocessor on

    4. (Microarchitecture, with the micro program counter set to the value of the instruction):

  4. Inc PC
  5. PC -> memory address register (MAR) [load the immediate address]
  6. Memory Data Register (MDR) -> MAR [Load from this immediate address]
  7. MDR -> R00

Sorry for the explanation... I may have gotten carried away. Anyway: I explain all this because I didn't realize until much later that I had build a complex instruction set; over the whole year of having nearly 2 hours of boring, useless lectures each day, each of this steps seemed almost inevitable. Like, how could I do more per clock cycle? I'd have to break down the instructions, and... one thing kinda led to another, if that makes sense.

"Re-reading, I dunno if that even answers your initial question, I just had fun talking about it :P"

It didn't answer their question... like at all I think. But it was fun! Sorry x3c

For those of you interested in the project, some other posts detailing it's current state can be found below! I haven't typed these up because they're all pretty minor, once I get a big update done I might add it up here though!