rvem: a RISC-V Emulator

So far, this is based on George Hotz' emulator here: https://github.com/geohot/twitchcore/blob/master/cpu.py

What I did was change the memory system from a flat 16kb array to a dictionary of 4kb pages to support load/stores on addresses >16kb, and implement system calls so that I can compile and run C programs with the RISC-V gnu toolchain that uses newlib.

It runs Doom using Doomgeneric (github.com/ozkl/doomgeneric), a framebuffer based portable version of Doom. After every frame, the Doom draw function executes ecall with a custom system call I made that takes a pointer to the framebuffer. The syscall draws it to the screen using Pygame.

The emulator runs slowly since it's implemented in Python. I measured it to execute ~100,000 instructions per second on my pc (an Intel Core i9-9900k).

Setup for Ubuntu WSL

I am using Ubuntu WSL on Windows. These same steps should work on Ubuntu native.

Grab a prebuilt risc-v cross compiler toolchain (or optionally build it yourself)

https://github.com/stnolting/riscv-gcc-prebuilt

cd /opt/riscv
wget https://github.com/stnolting/riscv-gcc-prebuilt/releases/download/rv32i-4.0.0/riscv32-unknown-elf.gcc-12.1.0.tar.gz
sudo tar -zxvf riscv32-unknown-elf.gcc-12.1.0.tar.gz -C /opt/riscv/
export PATH=/opt/riscv/bin:$PATH

Get the risc-v tests and build them with the toolchain

git submodule update --init --recursive
cd riscv-tests
autoupdate
autoconf
./configure --target=riscv32-unknown-elf CC=/opt/riscv/bin/riscv32-unknown-elf-gcc
make -j

Make sure /opt/riscv/bin is in your path before running make -j. I don't know why it doesn't just use CC, but it didn't seem to work without passing CC either.

Run a test

cd custom_tests
./build.sh rsort/rsort
cd ../emul
python3 rvem.py ../custom_tests/rsort/rsort