64-bit Bare-Metal UEFI Hello World

This is a minimal, from-scratch 64-bit UEFI application (our "kernel") that prints "Hello World" directly using UEFI services, without external libraries like gnu-efi.

How it works

1. uefi.h: Defines the absolutely essential data types (EFI_SYSTEM_TABLE, EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL, etc.) required by the UEFI specification.
2. main.c: The entry point efi_main. It uses the SystemTable passed by the firmware to write strings directly to the screen via ConOut->OutputString.
3. Makefile: Uses clang and lld-link to build a PE32+ .efi executable (the format required by UEFI firmware).

Booting from a USB Drive

We have compiled the kernel and placed it at EFI/BOOT/BOOTX64.EFI. This is the default fallback boot path on 64-bit UEFI systems.

To create the bootable USB drive (on Linux):

1. Insert your USB drive (e.g., /dev/sdX).
2. Format the drive with a FAT32 filesystem. You can use tools like GParted or mkfs.fat -F 32 /dev/sdX1.
3. Mount the USB drive.
4. Copy the entire EFI directory generated here to the root of the USB drive. The structure should look exactly like this:

   [USB Root]
   └── EFI
       └── BOOT
           └── BOOTX64.EFI
   

5. Unmount the drive, plug it into your modern PC, enter the BIOS/UEFI boot menu, and select the USB drive!

Testing in QEMU (if installed)

If you have QEMU and OVMF (UEFI firmware for QEMU) installed, you can test it virtually:

qemu-system-x86_64 -bios /usr/share/OVMF/OVMF_CODE.fd -drive file=fat:rw:.,format=raw