UEFI
Introduction
(U)EFI or (Unified) Extensible Firmware Interface is a specification that defines a software interface between the operating system(s) and the platform's firmware. In the mid 90s Intel was creating a new processor architecture that was 64-bit, but wasn't backwards-compatible with the old x86. This architecture was the Itanium 64. Because the IA-64 only supports 64-bit instructions, the PC BIOS couldn't be used, therefore Intel developed the EFI specification. Later on this specification was managed (and still is) by the UEFI board, an association of several companies such as AMD, Microsoft, Intel, Apple and so on.
Support
The (U)EFI specification is currently defined for the Itanium platform, the x86-platform (64-bit included) and the ARM platform. Since Apple's movement with Mac OS X from the PowerPC architecture to the x86-architecture, they have been using EFI, although with minor differences here and there. Most modern operating systems, boot loaders and boot managers support (U)EFI as well nowadays. Most 64-bit versions of Microsoft Windows, FreeBSD, Apple Mac OS X and Linux all have support for UEFI to some extent. As for boot loaders and managers you generally have (E)GRUB and ELILO. As for emulators and virtual machines, you can generally use VirtualBox and Qemu.
Developing for (U)EFI
One can develop applications, boot loaders or drivers for (U)EFI if he/she has the required hardware of software to develop for it. As said before you can use VirtualBox and Qemu as both of them seem to have a (U)EFI implementation of some sort. Another option is to use Intel's TianoCore, which is supposedly their own (U)EFI firmware interface implementation to run on top of the PC BIOS and/or as a Coreboot payload. The simplest way is by getting the right hardware. Generally there are several motherboards available that have an (U)EFI implementation, sometimes with a PC BIOS implementation next to it. Below is a list of noticeable BIOS companies:
- AMI (Aptio).
- Phoenix (SecureCore, TrustedCore, AwardCore).
- Insyde (InsydeH20).
Tools
The (U)EFI Development Kit, the (U)EFI Toolkit and the (U)EFI specifications might be interesting to use. As for writing actual (U)EFI software you can use several compiles such as gcc and Microsoft Visual Studio for C and FASM for Assembly. The (U)EFI Toolkit might be interesting when using a C compiler, as it contains several C/C++ headers.
General Partition Table
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Binary Format
(U)EFI generally uses the PE-executable format, with its very own subtypes. Every (U)EFI application is basically a DLL without symbol tables et al, and another subtypes:
- (U)EFI application (10).
- (U)EFI boot service driver (11).
- (U)EFI run-time driver (12).
Example in C
Below is an example of an EFI application written in C that displays: "Hello World".
#include <efi.h>
EFI_STATUS main(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
{
SystemTable->ConOut->OutputString(SystemTable->ConOut, L"Hello World\r\n");
return EFI_SUCCESS;
}
Example in FASM
Below is an example of an EFI application written in x86 assembly (to be assembled with FASM) that displays: "Hello World".
format pe64 dll efi
entry main
section '.text' code executable readable
include 'efi.inc'
main:
sub rsp, 4 * 8 ; Reserve space for four arguments.
mov [Handle], rcx ; ImageHandle
mov [SystemTable], rdx ; Pointer to SystemTable.
lea rdx, [_hello]
mov rcx, [SystemTable]
mov rcx, [rcx + EFI_SYSTEM_TABLE.ConOut]
call [rcx + SIMPLE_TEXT_OUTPUT_INTERFACE.OutputString]
add rsp, 4 * 8
mov eax, EFI_SUCCESS
retn
section '.data' data readable writeable
Handle dq ?
SystemTable dq ?
_hello du 'Hello World',13,10,0
section '.reloc' fixups data discardable