Lx32-fpga

lx32

I built a computer from scratch because ChatGPT told me "no hay huevos." So here we are.

Custom processor, custom PCB, custom compiler. The whole thing.

ok first let me flex

Zero violations. Zero unconnected items. I cried a little.

(also this is the 5th time I rewrote this README. if it still looks AI-generated I give up)

what is this actually

lx32 is a 32-bit CPU with a custom instruction set, running on a custom PCB, compiled by a custom LLVM backend. You can write C, compile it, and it runs on simulation (for now).

I wanted to actually understand how computers work, not just read about it. So I built one.

The board is 80x60mm, about the size of a credit card. Black soldermask, gold finish.

the board

Here's what it looks like:

The schematics:

Measurements:

Long:

Width:

Full view:

The main chip is a Lattice iCE40HX4K FPGA — that's what becomes the processor. I chose it because the toolchain is fully open source (Yosys, nextpnr, icepack).

The rest of the board is USB-C for power and programming, SPI flash so the bitstream loads on power-up, two LDOs for the separate 3.3V and 1.2V rails, a USB-UART bridge to talk to it from my laptop, 32KB of external SRAM, and a VGA header. There's also an OLED that shows register values and the current instruction live, which is the coolest part honestly.

3D render, front:

3D render, back:

.STEP file if you want it: lx32-fpga.step

the processor

32 registers, fixed 32-bit instruction width, single-cycle. I made up the instruction encoding: ADD, SUB, AND, OR, XOR, shifts, loads, stores, branches, jumps, LUI, AUIPC. x0 is hardwired to zero. Everything is in rtl/ and every module has a spec in docs/rtl/.

how I know it works

I wrote a Rust model of the processor, then a fuzzer that fires random instruction sequences through both Verilator and the model at the same time and compares cycle by cycle.

Runs in under 75 seconds. I also tortured my laptop with a Python script to run it on loop. There are Coq proofs for some properties and formal verification through sby too.

the compiler

LLVM backend for lx32 — instruction patterns, register file, calling convention. Eight programs compile and run end-to-end: return, pointer store, call chain, branch/loop, compare/assign, pointer walk, iterative fibonacci, recursive fibonacci.

Write C, compile with LLVM, runs on this board.

the gold art

ENIG means the exposed copper comes out gold on black soldermask. I put personal stuff on both sides.

The back has an infinity symbol, pototo ralora arerita, "All we need is love", Lizzie <3, 22/03/09, and 67 (that last one I didn't choose, it just ended up there).

It's a working computer and it also has my whole heart on it.

run it yourself

git clone https://github.com/looprotate/lx32.git
cd lx32
make setup

You'll need: verilator, Rust (cargo), coqc, sby, yosys, z3, g++.

make sim TB=lx32_system_tb    # full system sim
make validate                  # full test suite (1.1B vectors)
make formal-all                # formal proofs

BOM

Full BOM: lx32-bom.csv

MIT