As a part of the OS pro­ject for the uni­ver­sity there has been a re­quest to also write up the ex­per­i­ences and chal­lenges en­countered. This is the first post of the series on writ­ing a x64 op­er­at­ing sys­tem when boot­ing straight from UE­FI. Please keep in mind that these posts are writ­ten by a not-even-hob­by­ist and con­tent in these posts should be taken with a grain of salt.

Ker­nel and UEFI

I’ve de­cided to write a ker­nel tar­get­ing x64 as an UEFI ap­plic­a­tion. There are a num­ber of reas­ons to write a ker­nel as an UEFI ap­plic­a­tion as op­posed to writ­ing a multi­boot ker­nel. Namely:

  1. For x86 fam­ily of pro­cessors, you avoid the work ne­ces­sary to up­grade from real mode to pro­tec­ted mode and then from pro­tec­ted mode to long mode which is more com­monly known as a 64-bit mode. As an UEFI ap­plic­a­tion your ker­nel gets a fully work­ing x64 en­vir­on­ment from the get-go;
  2. Un­like BIOS, UEFI is a well doc­u­mented firm­ware. Most of the in­ter­faces provided by BIOS are de facto and you’re lucky if they work at all, while most of these provided by UEFI are de jure and usu­ally just work;
  3. UEFI is ex­tens­ible, whereas BIOS is not really;
  4. Fi­nally, UEFI is a mod­ern tech­no­logy which is likely to stay around, while BIOS is a 40 years old piece of tech­no­logy on the death row. Learn­ing about soon-to-be-dead tech­no­logy is a waste of the ef­fort.

Des­pite my strong at­tach­ment to the Rust com­munity and Rust’s per­fect suit­ab­il­ity for ker­nels1, I’ll be writ­ing the ker­nel in C. Mostly be­cause of how un­likely it is for people at the uni­ver­sity to be fa­mil­iar with Rust. Also be­cause GNU-EFI is a C lib­rary and I can­not be bothered to bind it. I’d surely be writ­ing it in Rust was I more ser­i­ous about the pro­ject.


As it turns out, de­vel­op­ing a x64 ker­nel on a x64 host greatly sim­pli­fies set­ting up the build tool­chain. I’ll be us­ing:

  • clang to com­pile the C code (no cross-­com­piler is ne­ces­sary2!);
  • gnu-efi lib­rary to in­ter­act with the UEFI firm­ware;
  • qemu emu­lator to run my ker­nel; and
  • OVMF as the UEFI firm­ware.

The UEFI “Hel­lo, world!”

The fol­low­ing snip­pet of code is all you need to print some­thing on the screen as an UEFI ap­plic­a­tion:

// main.c
#include <efi.h>
#include <efilib.h>
#include <efiprot.h>

efi_main (EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
    InitializeLib(ImageHandle, SystemTable);
    Print(L"Hello, world from x64!");
    for(;;) __asm__("hlt");

However, com­pil­ing this code cor­rectly is not as trivi­al. Fol­low­ing three com­mands are ne­ces­sary to pro­duce a work­ing UEFI ap­plic­a­tion:

clang -I/usr/include/efi -I/usr/include/efi/x86_64 -I/usr/include/efi/protocol -fno-stack-protector -fpic -fshort-wchar -mno-red-zone -DHAVE_USE_MS_ABI -c -o src/main.o src/main.c
ld -nostdlib -znocombreloc -T /usr/lib/ -shared -Bsymbolic -L /usr/lib /usr/lib/crt0-efi-x86_64.o src/main.o -o -lefi -lgnuefi
objcopy -j .text -j .sdata -j .data -j .dynamic -j .dynsym  -j .rel -j .rela -j .reloc --target=efi-app-x86_64 huehuehuehuehue.efi

The clang com­mand is pretty self-­ex­plan­at­ory: we tell the com­piler where to look for the EFI head­ers and what to com­pile into an ob­ject file. Prob­ably the most non-trivial op­tion here is the -DHAVE_USE_MS_ABI – x64 UEFI uses the Win­dows’ x64 call­ing con­ven­tion, and not the reg­u­lar C one, thus all ar­gu­ments in calls to UEFI func­tions must be passed in a dif­fer­ent way than it is usu­ally done in C code. His­tor­ic­ally this con­ver­sion was done by the uefi_call_wrapper wrap­per, but clang sup­ports the call­ing con­ven­tion nat­ively, and we tell that to the gnu-efi lib­rary with this op­tion3.

Then, I manu­ally link my ob­ject file and UE­FI-spe­cific C runtime up into a shared lib­rary us­ing a cus­tom linker script provided by the gnu-efi lib­rary. The res­ult is an ELF lib­rary about 250KB in size. However, UEFI ex­pects its ap­plic­a­tions in PE ex­ecut­able form­at, so we must con­vert our lib­rary into the de­sired format with the objcopy com­mand. At this point huehuehuehuehue.efi file should be pro­duced and ma­jor­ity of UEFI firm­wares should be able to run it.

In prac­tice, I’ve auto­mated these steps along with a con­sid­er­ably com­plex se­quence of build­ing im­age files I’ve stolen from OS­DEV’s tu­torial on cre­at­ing im­ages into a Make­file. Feel free to copy it in parts or in whole for your own use cases.

UEFI boot and runtime ser­vices

An UEFI ap­plic­a­tion has 2 dis­tinct stages over its life­time: a stage where so-c­alled boot ser­vices are avail­able and stage after these boot ser­vices are dis­abled. An UEFI ap­plic­a­tion will be launched by the UEFI firm­ware and both boot and runtime ser­vices will be avail­able to the ap­plic­a­tion. Most not­ably, boot ser­vices provide APIs for load­ing other UEFI ap­plic­a­tions (e.g. to im­ple­ment boot­load­er­s), hand­ling (al­loc­at­ing and deal­loc­at­ing) memory and us­ing pro­to­cols (speak­ing to other act­ive UEFI ap­plic­a­tion­s).

Once the ker­nel is done with us­ing boot ser­vices it calls ExitBootServices which is a method provided by… a boot ser­vice. Past that point only runtime ser­vices are avail­able and you can­not ever re­turn to a state where boot ser­vices are avail­able ex­cept by re­set­ting the sys­tem. Man­aging UEFI vari­ables, sys­tem clock and re­set­ting the sys­tem is pretty much the only things you can do with the runtime ser­vices.

For my ker­nel, I will use the graph­ics out­put pro­tocol to set up the video frame buf­fer, exit the boot ser­vices and, fi­nally, shut down the ma­chine be­fore reach­ing the hlt in­struc­tion. Fol­low­ing piece of code im­ple­ments the de­scribed se­quence. I left some code out, you can see it in full at Git­lab. For ex­ample, the defin­i­tion of init_graphics.

efi_main (EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
    EFI_STATUS status;
    InitializeLib(ImageHandle, SystemTable);

    // Initialize graphics
    status = SystemTable->BootServices->LocateProtocol(
        &graphics_proto, NULL, (void **)&graphics
    if(status != EFI_SUCCESS) return status;
    status = init_graphics(graphics);
    if(status != EFI_SUCCESS) return status;

    // Figure out the memory map (should be identity mapping)
    boot_state.memory_map = LibMemoryMap(
    // Exit the boot services...
        ImageHandle, boot_state.map_key
    // and set up the memory map we just found.
    // Once we’re done we power off the machine.
        EfiResetShutdown, EFI_SUCCESS, 0, NULL
    for(;;) __asm__("hlt");

Note, that some pro­to­cols can either be at­tached to your own EFI_HANDLE or some other EFI_HANDLE (i.e. pro­tocol is provided by an­other UEFI ap­plic­a­tion). Graph­ics out­put pro­tocol I’m us­ing here is an ex­ample of a pro­tocol at­tached to an­other EFI_HANDLE, there­fore we use LocateProtocol boot ser­vice to find it. In the off-chance a pro­tocol is at­tached to the ap­plic­a­tion’s own EFI_HANDLE, the HandleProtocol method should be used in­stead:

EFI_LOADED_IMAGE *loaded_image = NULL;
EFI_GUID loaded_image_protocol = LOADED_IMAGE_PROTOCOL;
EFI_STATUS status = SystemTable->BootServices->HandleProtocol(
    ImageHandle, &loaded_image_protocol, &loaded_image

Next steps

At this point I have a bare bones frame for my awe­some ker­nel called “hue­hue­hue­hue­hue”. From this point on­wards the de­vel­op­ment of the ker­nel should not dif­fer much from the tra­di­tional de­vel­op­ment of any other x64 ker­nel.

  1. As proved by nu­mer­ous ker­nels writ­ten in Rust out there.↩︎

  2. That be­ing said, clang can cross-­com­pile without hav­ing to build it from source for that pur­pose in the first place, like it is ne­ces­sary to do with gcc.↩︎

  3. If gnu-efi was a per­fect lib­rary, the -DGNU_EFI_USE_MS_ABI op­tion should be used in­stead, but the lib­rary only ver­sion-checks for gcc and al­ways re­ports that clang does not sup­port the op­tion.↩︎