os-dev-scratch

OS Development from Scratch

Purpose

Guide agents through building a minimal operating system from scratch: bootloader stages (BIOS/GRUB vs UEFI/limine), 64-bit long mode setup with GDT and page tables, IDT and interrupt handlers, PIC/APIC configuration, basic keyboard and serial drivers, physical and virtual memory managers, context switching, with xv6-RISC-V as a reference architecture.

When to Use

• Learning how an OS boots from power-on to main()
• Implementing protected/long mode transitions on x86-64
• Writing a physical memory allocator (bitmap) and page table manager
• Handling timer, keyboard, and page fault interrupts
• Testing with QEMU -kernel and cross-compiler x86_64-elf-gcc
• Porting concepts from xv6 to a custom x86 or RISC-V kernel

Workflow

1. Boot stages overview

BIOS path (legacy)
├── BIOS POST
├── MBR (512 bytes) → boot sector loads stage2
├── GRUB/multiboot → loads kernel ELF
└── kernel entry (_start)

UEFI path (modern)
├── UEFI firmware
├── EFI bootloader (limine, systemd-boot)
├── Loads kernel + initrd from ESP
└── kernel entry (handoff with memory map)

2. Toolchain setup

# Cross-compiler for bare metal
brew install x86_64-elf-gcc x86_64-elf-binutils   # macOS
# or build from source / apt install gcc-x86-64-elf

x86_64-elf-gcc --version

# QEMU for testing
qemu-system-x86_64 --version

Linker script essentials:

/* linker.ld */
ENTRY(_start)
SECTIONS {
    . = 0x100000;          /* 1MB — typical kernel load address */
    .text : { *(.text .text.*) }
    .rodata : { *(.rodata .rodata.*) }
    .data : { *(.data .data.*) }
    .bss : { *(.bss .bss.*) }
}

x86_64-elf-gcc -ffreestanding -nostdlib -c kernel.c -o kernel.o
x86_64-elf-ld -T linker.ld kernel.o -o kernel.elf

3. Multiboot/limine boot

# QEMU direct kernel boot (no disk)
qemu-system-x86_64 \
  -kernel kernel.elf \
  -serial stdio \
  -m 128M \
  -no-reboot -no-shutdown

# With limine (UEFI)
qemu-system-x86_64 \
  -bios /usr/share/ovmf/OVMF.fd \
  -drive file=disk.img,format=raw \
  -serial stdio