PseudOS is a toy operating system based on the following resources:

• https://www.youtube.com/watch?v=s_4tFz52jbc
• xv6

PseudOS runs on QEMU and needs a riscv64 toolchain to compile. See https://www.qemu.org/docs/master/system/riscv/virt.html for more details

Running make should take care of everything

See the makefile for compilation details

Again, make should automatically run the OS. If you're constantly iterating on the code (like I am) and QEMU doesn't auto-quit on OS crash, I've included a qemu destroyer bash script which can run in another terminal tab and continuously pkills qemu. Just a quality of life thing.

If you want to inspect QEMU to see what's going on under the hood, use the following commands:

First, run

make pseudOS_debug

which will compile the OS with debugging symbols on and no optimizations. The -S command in the makefile will hold QEMU on startup and not execute any code until we've connected with gdb.

In a separate terminal window, start gdb: riscv64-unknown-elf-gdb build/kernel.elf

Connect to the QEMU session with the command

(gdb) target remote localhost:9000

Use this command to activate GDB's display of source and registers

(gdb) tui reg general

Skip to some interesting code with with

(gdb) advance _move_to_supervisor_mode

Get the status of specific CSR registers

(gdb) info registers mstatus

(Optional) Set up gdbgui (https://www.gdbgui.com/) to have a GUI not from the 80s. It also supports vim keybindings for those who prefer it to gdb's emacs bindings.

/usr/local/bin/python3.9 -m pipx run gdbgui

Ref:

• https://spivey.oriel.ox.ac.uk/compilers/Single-stepping_with_QEMU_and_GDB
• https://jvns.ca/blog/2021/05/17/how-to-look-at-the-stack-in-gdb/