sudo apt-get install build-essential gnu-efi qemu

# Run the default program on QEMU.
make run

# Run a given program.
make run RUN=min
make run RUN=bios_one_char

Tested on Ubuntu 14.04.

Run on real hardware: for insert as USB, determine its device (/dev/sdX) with:

sudo lsblk
sudo fdisk -l

then run:

sudo dd if=bios_hello_world.img of=/dev/sdX

Then:

insert the USB in a computer
during boot, hit some special key, usually F12
choose to boot from the USB

Tested on: ThinkPad T400.

More assembly info at: https://github.com/cirosantilli/assembly-cheat

Formats

When we create a regular Linux program, we generate an ELF file, which is read by the OS.

Without an OS, we can use the following formats:

boot sector. TODO where is it specified, if at all? Wiki page describes MBR well enough: https://en.wikipedia.org/wiki/Master_boot_record
multiboot. Defined by GRUB. More boilerplate, but much more convenient.

Gotchas

bytes 511 and 512 of the boot sector must be 0x55aa or else the BIOS will refuse to load
BIOS loads the program into memory at the address 0x7C00.

We must tell that magic number to the linker somehow, either with a linker script, -tText=-Ttext 0x7C00 or NASM org 0x7c00.

This will only matter when you access a memory address, because of relocation.

If you don't know what relocation is, first read this: http://stackoverflow.com/questions/12122446/how-does-c-linking-work-in-practice/30507725#30507725

When we link a normal program with an OS, the linker tells where it wants the OS to place it in virtual memory.

But for the boot sector, the BIOS puts the program into memory. So we must tell that to the linker somehow. Otherwise it cannot know what addresses to use for instructions.
x86 processors start in 16-bit mode.

IO

You cannot use any libraries, so how to do IO? Some ways that this can be done:

BIOS functions: http://wiki.osdev.org/BIOS. Not well standardized like it's successor UEFI. Called through interrupts.
https://en.wikipedia.org/wiki/VGA-compatible_text_mode
VBE https://en.wikipedia.org/wiki/VESA_BIOS_Extensions

Showdown and restart can be managed with either:

ACPI https://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface

Newer and better.

Now managed by the same group that manages UEFI.

Spec:
- current: http://uefi.org/specifications
- old: http://www.uefi.org/acpi/specs
APM https://en.wikipedia.org/wiki/Advanced_Power_Management

http://wiki.osdev.org/APM

Older and simpler.

By Microsoft in 1995. Spec seems to be in RTF format...

Bibliography

http://stackoverflow.com/questions/22054578/run-a-program-without-an-operating-system
https://github.com/programble/bare-metal-tetris tested on Ubuntu 14.04. Just works.

Has Multiboot and El Torito. Uses custom linker script.

Almost entirely in C -nostdlib, with very few inline asm commands, and a small assembly entry point. So a good tutorial in how to do the bridge.
osdev.org is a major source for this.
https://github.com/scanlime/metalkit A more automated / general bare metal compilation system. Untested, but looks promising.

The following did not work on my machine out of the box: