“Hello, OVMF!” in NASM

This repository demonstrates the assembly of a minimal UEFI Image on Linux using only make and NASM. The UEFI Image targets OVMF on the x86_64 architecture.

Prerequisites

The programs make, NASM and QEMU are needed to build and run the UEFI Image.

The files OVMF_CODE.fd and OVMF_VARS.fd, which may be symbolic links, are expected to exist in the directory at /usr/share/OVMF.

Usage

The following shell command will prepare the repository, assemble the UEFI Image and launch a VM to run the image:

$ make setup && make run

The expected console output follows:

UEFI Interactive Shell v2.2
EDK II
UEFI v2.70 (EDK II, 0x00010000)
Mapping table
      FS0: Alias(s):HD0a1:;BLK1:
          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)
     BLK0: Alias(s):
          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
     BLK2: Alias(s):
          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Press ESC in 1 seconds to skip startup.nsh or any other key to continue.
Shell> FS0:\hello.efi
Hello, OVMF!
Shell> 

To exit QEMU, press Ctrl+A and then X.

PE32+ header construction

The DOS Stub and the data directories in the optional header aren’t needed and are therefore omitted.

Some header fields, e.g., MajorSubsystemVersion, are populated for only documentation purposes.

The stack and heap reserve and commit sizes are chosen to match the respective default values for the MSVC Linker.

The image base of 0x100000 is chosen to match the first available address in the VM’s memory map that meets the application’s memory requirements. (This map can be obtained by running memmap -b in the UEFI Shell.)

The choice of image base renders relocation unnecessary. There is therefore no base relocation table and the file header is made to indicate that relocations have been stripped. The image can nonetheless be relocated because it comprises position-independent code. To see relocation in action:

1. Change the image base to an unavailable address, e.g., 0x76ed000
2. Unset the IMAGE_FILE_RELOCS_STRIPPED bit in the file header characteristics
3. Re-assemble and rerun the image

License

This project comprises free and open-source software and documentation licensed under the MIT license.