A from-scratch OS for RISC-V 64-bit

Note: The source code and comments are currently in French but are being progressively translated.

Implemented Features

0. RAM Structure

• Structure: Linker script kernel.lds

1. Kernel & Bootstrapping

• Boot: Initialization via crt0.S and enter_smode.S then calling kernel_start (C code)
• Privilege Modes: Transition from Machine mode to Supervisor mode; execution happens in User mode (and S-mode for idle).

2. Memory Management

• Paging (Virtual Memory): Implementation of the Sv39 standard with a 3-level page table (page walk).
• Isolation: Separation of kernel and user address spaces (Segfault errors are triggered).
• Physical Memory Allocation: Frame allocator using a free list.
• Heaps (Buddy System Algorithm):
  • kheap: Dynamic heap for the kernel.
  • uheap: Dynamic heap for user processes.

3. Processes

• Scheduler: Preemptive Round-Robin scheduler.
• Process Management:
  • States: RUNNING, READY, SLEEPING, ZOMBIE, WAITING.
  • Primitives: create_process, waitpid, sleep, ps.
  • idle process: Executed when no other process is ready (uses wfi).
• Context Switching: Saving and restoring registers via ctx_sw.

4. Interrupts & Exceptions

• Trap Handler:
  • Centralized management of exceptions (syscalls and page faults) and interrupts (Timer, UART interrupts raised by the PLIC).
  • Switching stacks (mscratch, sscratch, sp)
• System Calls: ecall interface allowing user processes to interact with the kernel (I/O, processes, FS).

5. File System (RAMFS)

• A simple file system residing entirely in RAM.
• Support for directories and files (WIP).
• Commands: mkdir, ls, pwd, cd, rm.

6. Drivers & I/O

• UART: Serial communication for logs and text-mode keyboard input.
• Screen (Framebuffer): Graphic support via bochs-display and PCIe with custom font rendering (font.c).

7. User Interface

• Shell (bash custom): CLI to navigate the FS and launch processes.
  • Commands: echo, clear, bash, ... & (background process), etc

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Getting Started

Prerequisites

• RISC-V Toolchain: gcc-riscv64-unknown-elf
• Emulator: qemu-system-riscv64
• Make: make
• GDB (optional): gdb-multiarch

Start the kernel

• Console mode (nographic):

  make run

• Graphic mode:

  make rung

Debugging

• Console mode (nographic):

  make debug

• Graphic mode:

  make debugg

Then open a new terminal and run:

gdb-multiarch kernel.bin -tui