a simple OS kernel, made at PAIR camp 2025 by max, zehao, and rohan

The Flow

Core Elements

• Makefile
• linker.ld
• kernel.c (C entry point that writes “Hello, world!” to the screen)
  • The entry point to a UEFI application is a C file, no other assembly files needed.

Process

1. Makefile
   • Defines how make compiles each .S and .c into .o object files.
   • Invokes the linker with linker.ld to lay out those objects in memory and set the entry point.
   • Packages the resulting kernel.efi into a bootable image (FAT-ESP).
2. linker.ld
   • Tells the linker where to place the code/data sections and which symbol is the entry point.
3. Linker (i686-elf-ld)
   • Takes all the .o files plus the script, resolves symbols, applies relocations, and produces the final EFI image (kernel.efi).
4. Boot Image
   • Creates a FAT image (esp.img) with EFI/BOOT/BOOTX64.EFI → done with QEMU with OVMF

Integration

[Our Code] <--> [UEFI] <--> [QEMU (simulated hardware)]

• UEFI runs our code which is stored in a bootx.esi file
• We use GNU-EFI library to call UEFI Boot Services from C.
  • UEFI Boot Services give rich APIs to do things like memory services without early bootloader codes.

Development Process

At the beginning, maybe pangea kinda time, we spent 10 minutes struggling with no c standard library, what the hell? It turns out, we imported the libraries the wrong way round. Next, what a beautiful error, there was missing library?! No, we missed the -c flag in our Makefile for approximately half an hour. Then, we were on the home stretch, but the InitializeLib function was missing :(. How? I swore I included it? Oh, a missing, underscore.