Simple 64-bit UEFI bootloader for the x86_64 architecture. Kernel must be a 64-bit ELF executable. Program segments are loaded at the virtual addresses specified in the program header table. Upon successful boot the processor jumps to the kernel's entry point and loads the BootInfo structure's virtual address in register %rdi (first function parameter according to the System V ABI). 4-level recursive paging is also set up by the bootloader.
BootInfo structure:
- efi_system_table physical address
- FrameBuffer structure
- width
- height
- pitch
- base (framebuffer physical address, format used is PixelBlueGreenRedReserved8BitPerColor)
- MemoryMap structure
- descriptor_size (size of EFI_MEMORY_DESCRIPTOR)
- size (size of the memory map)
- map (physical address of the list of EFI_MEMORY_DESCRIPTORS)
The messages displayed by the bootloader can be configured using the following CMake options:
- basic_logging: activates simple messages
- extensive_logging: activates messages from internal functions
- print_elf: prints elf headers
- print_memory_map: prints initial memory, it can change
- print_tokens: prints config file tokenization(only if built in Debug mode)
- print_config: prints chosen bootloader config
The bootloader is capable of loading other configurable files, called initial datafiles. An example bootloader configuration can be found in boot.cfg. The loader will search for the configuration in /config/boot.cfg.
Segements loaded from the kernel image, page tables, initial data files and the BootInfo are all marked as EfiReservedMemoryType. The bootloader allocates page tables such that the first 2 MiB of the virtual address space is mappable, meaning one can map the first 512 4 KiB pages without checking the presence of the higher level page table entries. This is done because at the beginning the kernel is not capable of memory allocation but it needs a way of mapping known physical addresses from the BootInfo structure without allocating additional page tables.
To change the compiler modify the Toolchain.cmake file. The built UEFI application can be found in build/efi.