Technically, a program that runs without an OS, is an OS. So let's see how to create and run some minuscule hello world OSes.
The code of all examples below is present on this GitHub repo. All was tested on Ubuntu 14.04 AMD64 QEMU and real hardware ThinkPad T430.
Here we create one with a single
printf '\364%509s\125\252' > main.img sudo apt-get install qemu-system-x86 qemu-system-x86_64 -hda main.img
main.img contains the following:
\364in octal ==
0xf4in hex: the encoding for a
hltinstruction, which tells the CPU to stop working.
Therefore our program will not do anything: only start and stop.
We use octal because
\xhex numbers are not specified by POSIX.
We could obtain this encoding easily with:
echo hlt > a.asm nasm -f bin a.asm hd a
0xf4encoding is also documented on the Intel manual of course.
%509sproduce 509 spaces. Needed to fill in the file until byte 510.
\125\252in octal ==
0xaa: magic bytes required by the hardware. They must be bytes 511 and 512.
If not present, the hardware will not treat this as a bootable disk.
Note that even without doing anything, a few characters are already printed on the screen. Those are printed by the firmware, and serve to identify the system.
Run on real hardware
Emulators are fun, but hardware is the real deal.
Burn the image to an USB stick (will destroy your data!):
sudo dd if=main.img of=/dev/sdX
plug the USB on a computer
turn it on
tell it to boot from the USB.
This means making the firmware pick USB before hard disk.
If that is not the default behavior of your machine, keep hitting Enter, F12, ESC or other such weird keys after power-on until you get a boot menu where you can select to boot from the USB.
It is often possible to configure the search order in those menus.
Now that we have made a minimal program, let's move to a hello world.
The obvious question is: how to do IO? A few options:
- ask the firmware, e.g. BIOS or UEFI, to do if for us
- VGA: special memory region that gets printed to the screen if written to. Can be used on Protected mode.
- write a driver and talk directly to the display hardware. This is the "proper" way to do it: more powerful, but more complex.
- use debug features of chips. ARM calls theirs semihosting for example. On real hardware, it requires some extra hardware and software support, but on emulators it can be a free convenient option. Example.
Here we will do a BIOS example as it is simpler. But note that it is not the most robust method.
Here is the GAS code:
.code16 .global _start _start: cli mov $msg, %si mov $0x0e, %ah loop: lodsb or %al, %al jz halt int $0x10 jmp loop halt: hlt msg: .asciz "hello world" .org 510 .word 0xaa55
Besides the standard userland assembly instructions, we have:
.code16: tells GAS to output 16-bit code
cli: disable software interrupts. Those could make the processor start running again after the
int $0x10: does a BIOS call. This is what prints the characters one by one.
.word 0xaa55: place the magic bytes at the end
A quick and dirty way to compile this is with:
as -o main.o main.S ld --oformat binary -o main.ing -Ttext 0x7C00 main.o
main.img as before.
There are two important flags here:
--oformat binary: output raw binary assembly code, don't warp it inside an ELF file as is the case for regular userland executables.
-Ttext 0x7C00: we need to tell the linker
ldwhere the code will be placed so that it will be able to access the memory.
In particular, this is used during the relocation phase. Read more about it here.
The better way of compiling is to use a clean linker script as this one. The linker script can also place the magic bytes for us.
In truth, your boot sector is not the first software that runs on the system's CPU.
What actually runs first is the so-called firmware, which is a software:
- made by the hardware manufacturers
- typically closed source but likely C-based
- stored in read-only memory, and therefore harder / impossible to modify without the vendor's consent.
Well known firmwares include:
- BIOS: old all-present x86 firmware. SeaBIOS is the default open source implementation used by QEMU.
- UEFI: BIOS successor, better standardized, but more capable, and incredibly bloated.
- Coreboot: the noble cross arch open source attempt
The firmware does things like:
loop over each hard disk, USB, network, etc. until you find something bootable.
When we run QEMU,
main.imgis a hard disk connected to the hardware, and
hdais the first one to be tried, and it is used.
load the first 512 bytes to RAM memory address
0x7c00, put the CPU's RIP there, and let it run
show things like the boot menu or BIOS print calls on the display
Firmware offers OS-like functionality on which most OS-es depend. E.g. a Python subset has been ported to run on BIOS / UEFI: https://www.youtube.com/watch?v=bYQ_lq5dcvM
It can be argued that firmwares are indistinguishable from OSes, and that firmware is the only "true" bare metal programming one can do.
As this CoreOS dev puts it:
The hard part
When you power up a PC, the chips that make up the chipset (northbridge, southbridge and SuperIO) are not yet initialized properly. Even though the BIOS ROM is as far removed from the CPU as it could be, this is accessible by the CPU, because it has to be, otherwise the CPU would have no instructions to execute. This does not mean that BIOS ROM is completely mapped, usually not. But just enough is mapped to get the boot process going. Any other devices, just forget it.
When you run Coreboot under QEMU, you can experiment with the higher layers of Coreboot and with payloads, but QEMU offers little opportunity to experiment with the low level startup code. For one thing, RAM just works right from the start.
Post BIOS initial state
Like many things in hardware, standardization is weak, and one of the things you should not rely on is the initial state of registers when your code starts running after BIOS.
So do yourself a favor and use some initialization code like the following: https://stackoverflow.com/a/32509555/895245
%es have important side effects, so you should zero them out even if you are not using them explicitly.
Note that some emulators are nicer than real hardware and give you a nice initial state. Then when you go run on real hardware, everything breaks.
Boot sectors are simple, but they are not very convenient:
- you can only have one OS per disk
- the load code has to be really small and fit into 512 bytes. This could be solved with the int 0x13 BIOS call.
- you have to do a lot of startup yourself, like moving into protected mode
It is for those reasons that GRUB created a more convenient file format called multiboot.
If you prepare your OS as a multiboot file, GRUB is then able to find it inside a regular filesystem.
This is what most distros do, putting OS images under
Multiboot files are basically an ELF file with a special header. They are specified by GRUB at: https://www.gnu.org/software/grub/manual/multiboot/multiboot.html
You can turn a multiboot file into a bootable disk with
Format that can be burnt to CDs: https://en.wikipedia.org/wiki/El_Torito_%28CD-ROM_standard%29
It is also possible to produce a hybrid image that works on either ISO or USB. This is can be done with
grub-mkrescue (example), and is also done by the Linux kernel on
make isoimage using
ARM-land has its own conventions (or more lack of conventions, since ARM licenses IP to vendors that modify it), but the general ideas are the same:
- you write code to a magic address in memory
- you do IO with magic addresses
Some differences include:
- IO is done by writing to magic addresses directly, there is no
- you need to add magic compiled closed source blobs provided by vendors to your image. Those are somewhat like BIOS, and that is a good thing, as it makes updating that firmware more transparent.
Here are some examples: