Welcome! Sounds like a fun project.

A IBM 360 clone sounds like more fun, than RISC.
Spend time to consider about hardware debugging, a front panel,and bus layouts, since you are building a computer system.
Manchester Baby is good example of a simple computer, to test out the system. Later the real computer can be built.

Hi I'm George, and I already know quite a few of you from other forums like 6502.org. I've joined here as I'm finding myself more and more interested in CPU design, and have recently gone down a rabbit hole designing my own ISA for a practically-useful design which I hope to be able to build one day.

Professionally I've always been a software engineer, though these days I don't do the actual engineering any more. Outside of professional work I grew up learning computing on the BBC Micro, including some 6502 assembly language, before moving to PCs programming in C and x86 assembler, getting involved with the DJGPP and Allegro projects, and moving most of my home computing to Linux.

However about five years ago I developed an interest in how computers actually work, being especially interested in some aspects of the BBC Micro which I grew up using but not understanding at that level, and it was an eye-opener how simple these things actually are compared to the complex black boxes I'd assumed them to be. I watched a lot of Ben Eater's SAP-1 videos, made some 6502-based computers based on Grant Searle's simple design, and started digging a lot more deeply into display circuits, all this mostly on solderless breadboards but sometimes making PCBs. I've made YouTube videos documenting some of those projects too.

These were mostly based on the 6502, though I also picked up an ARM2 processor and made most of a computer using that. Those 32-bit buses though... lots of wires! I hope to get back to that one at some point but I am easily distracted, and not ashamed to let things lie abandoned for a while sometimes. There's only so much time in the day alongside work and hobbies need to be fun diversions, not a burden or obligation.

I recently took a long break from all of this, and one distraction in that period was trying to improve at chess. This year I started playing in some tournaments and got some official ratings, and I'm happy with those even if they're at the low end of the scale! Realistically it is hard to improve significantly as an adult but I've enjoyed the process and will try to keep it going on the back burner at least.

Over the past month or two my computer hardware interests have reasserted themselves, and among them is this CPU design which I've suddenly taken a lot further than I had expected to - it went from designing a simple ISA based on my hardware ideas, to making it be a 16-bit subset of RV32, to writing microcode to check that it can be implemented with a simple enough core design; and then I wanted to check that it was actually useful to program with this ISA (since I'm not familiar with RISCV at all), so I wrote an assembler and simulator for it, and now I've ended up also writing a lot of standard library code for it too, like I/O, string handling, malloc/free, mostly following the standard C APIs for those. I'm constantly surprised by how easy it is to work with, in spite of the limitations required to fit everything in 16-bit instructions.

So it's one of those projects that somehow seems to develop itself, and I'm surprised by how much momentum it's had. It's got to a point where I wanted to look more widely at other people's projects, and see what the "done thing" is, and if I'm making glaring mistakes - so I thought I'd join up here and have a look around!

If anybody is interested in more details on the CPU project let me know and maybe I can post more information, I haven't really thought through how to communicate any of it yet though and none of the code is in github yet.

Welcome George, good to see you here! And thanks for the bit of personal history.

It is quiet here, but we do still see a number of regular visitors, and quite a few interested in esoteric or homebuilt CPUs.

Edit: would certainly like to see any and all details of your thoughts so far. Probably best in
Projects
or perhaps
General Discussions

Edit: I see a few like-minded threads also in
CPU/MCU choices and designs

Thanks Ed. I have put where I am so far into a github repository at least! https://github.com/gfoot/gf-rv16 I will try to post more coherent details soon though!