os

• Architecture Target: 64-bit ARMv8, i.e. aarch64/arm64
• Hardware Target: Michelle's Pi 3 Model B
• RAM: 1GB

Build Dependencies

• Zig Compiler
• LLVM for llvm-objdump and lldb
• QEMU for running in a virtual machine

Useful Commands

• Dump out the kernel ELF file:

  llvm-objdump --arch-name=aarch64 -D ./zig-out/bin/kernel.elf > ./obj.txt
  
  # Or use Zig:
  zig build dump
  

• Run the kernel in QEMU:

  qemu-system-aarch64 -M raspi3b -serial null -chardev stdio,id=uart1 -serial chardev:uart1 -kernel ./zig-out/bin/kernel8.img
  
  # Or use Zig:
  zig build run
  

Useful Resources

• Pi 3 Tutorial - https://github.com/s-matyukevich/raspberry-pi-os
• Another Pi 3 Tutorial - https://github.com/bztsrc/raspi3-tutorial/
• Aarch64 Calling Convention - https://hev.cc/3052.html
• Aarch64 barriers - https://developer.arm.com/documentation/100941/0101/Barriers
• Aarch64 shareability - https://developer.arm.com/documentation/den0024/a/Memory-Ordering/Memory-attributes/Cacheable-and-shareable-memory-attributes
• Aarch64 Address Translation Stage (different from levels) - https://developer.arm.com/documentation/102142/0100/Stage-2-translation
• Buddy allocator example code - https://github.com/a2liu/dumboss/blob/main/kern/memory.c
• Pi General - https://wiki.osdev.org/Raspberry_Pi_Bare_Bones
• Pi 4 - https://www.rpi4os.com/
• Quick-reference for instructions - https://courses.cs.washington.edu/courses/cse469/19wi/arm64.pdf
• Some maybe-useful ARMv8 info - https://m.youtube.com/watch?v=6OfIzhuw1RE
• Frame buffer - https://forums.raspberrypi.com/viewtopic.php?t=155825
• Ask GPU where its memory is - https://forums.raspberrypi.com/viewtopic.php?t=209388

Doc Sources

• RPi Model 3B+ Peripherals Manual.pdf - https://github.com/raspberrypi/documentation/files/1888662/BCM2837-ARM-Peripherals.-.Revised.-.V2-1.pdf
• ARM Clang Reference Manual.pdf - https://documentation-service.arm.com/static/5f0db3ca62665459ec77bc02
• ARMv8 Reference Manual.pdf - https://documentation-service.arm.com/static/60119835773bb020e3de6fee
• RPi Beginner Guide.pdf - https://magpi.raspberrypi.com/books/beginners-guide-4th-ed

Code Organization

Concept	Location
Boot-up	src/boot.S
System Timer	src/interrupts.zig
Interrupt Vectors	src/interrrupts.zig
Page Table Init	src/boot.S
MMU Protections	src/memory.zig
Page Allocator	src/memory.zig
Task Scheduler	src/scheduler.zig
Mini-UART Handler	src/mmio.zig
Linker Script	src/link.ld
Frame Buffer	src/framebuffer.zig
Mailboxes	src/mmio.zig

To Do

• Safety
  • Fix timer overflow
• Allocator
  • safety checks in allocator
  • handle GPU memory addresses in the allocator
• Debugging
  • switch from mini-uart to uart
  • receive data in UART
  • Debug shell
• Userspace Exe
  • remove identity mapping
  • FAT32 handler
  • syscalls and user-space executables
  • page mapping utilities
  • various IPC queues at different levels of overhead
• File storage as an extension of 64-bit memory
  • The kernel sees file data as basically just an extended page table
  • We can build file tree stuff on top, but the underlying implementation allows you to access pages of a file at a time by simply accessing memory normally.
• User-space optimization of memory? Compression, paging, etc.