RISCMON

A minimal machine monitor for RISC-V systems, inspired by WozMon.

About

Like Steve Wozniak's 256-byte monitor for the Apple I, Riscmon provides nothing but the essentials:

• Memory inspection
• Memory modification
• Jump to address and execute

No filesystem. No processes. No abstractions. Raw access to the machine.

Features

• UART serial I/O REPL
• Memory Read/Write
• Memory Dump (with formatting options)
• Jump and Execute

Core Commands

Command	Description
@	Print current address
@ADDR	Set current address
ADDR.ADDR	Dump memory range (inclusive)
ADDR+OFF	Dump OFF bytes starting at ADDR
ADDR.ADDR.as_str	Dump range as ASCII (non-printables shown as .)
ADDR+OFF.as_str	Dump OFF bytes as ASCII
ADDR: XX YY ...	Write up to 32 bytes starting at ADDR (tokens are hex, and may be aa or 0xaa)
jump ADDR	Jump to ADDR and execute (returns to REPL if callee returns)

Development

• Language: Rust (no_std) with minimal RISC-V assembly for boot
• Target: RISC-V 64-bit (riscv64gc-unknown-none-elf)
• Platform: QEMU virt machine (real hardware later)
• Interface: Serial console (UART)

Warn: ONLY a 64bit RISC-V QEMU system is supported currently.

Building

# Install RISC-V target
rustup target add riscv64gc-unknown-none-elf

# Build
cargo build --release

# Run in QEMU
qemu-system-riscv64 -machine virt -nographic -bios none -kernel target/riscv64gc-unknown-none-elf/release/riscmon

Or, simply run:

make run

Philosophy

Riscmon was built as an exercise, for fun. It answers the question: what is the absolute minimum a computer needs to be programmable?

The answer: memory access and a jump instruction.

Contributions

Fixes welcome. For new features create an issue first.

License

MIT