Runnable examples

Let's create and run some minuscule bare metal hello world programs that run without an OS on:

an x86 Lenovo Thinkpad T430 laptop with UEFI BIOS 1.16 firmware

an ARM-based Raspberry Pi 3

We will also try them out on the QEMU emulator as much as possible, as that is safer and more convenient for development. The QEMU tests have been on an Ubuntu 18.04 host with the pre-packaged QEMU 2.11.1.

The code of all x86 examples below and more is present on this GitHub repo.

How to run the examples on x86 real hardware

Remember that running examples on real hardware can be dangerous, e.g. you could wipe your disk or brick the hardware by mistake: only do this on old machines that don't contain critical data! Or even better, use cheap semi-disposable devboards such as the Raspberry Pi, see the ARM example below.

For a typical x86 laptop, you have to do something like:

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.

For example, on my T430 I see the following.

After turning on, this is when I have to press Enter to enter the boot menu:

Then, here I have to press F12 to select the USB as the boot device:

From there, I can select the USB as the boot device like this:

Alternatively, to change the boot order and choose the USB to have higher precedence so I don't have to manually select it every time, I would hit F1 on the "Startup Interrupt Menu" screen, and then navigate to:

Boot sector

On x86, the simplest and lowest level thing you can do is to create a Master Boot Sector (MBR), which is a type of boot sector, and then install it to a disk.

Here we create one with a single printf call:

printf '\364%509s\125\252' > main.img sudo apt-get install qemu-system-x86 qemu-system-x86_64 -hda main.img

Outcome:

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.

And on the T430 we just get a blank screen with a blinking cursor:

main.img contains the following:

\364 in octal == 0xf4 in hex: the encoding for a hlt instruction, which tells the CPU to stop working. Therefore our program will not do anything: only start and stop. We use octal because \x hex numbers are not specified by POSIX. We could obtain this encoding easily with: echo hlt > a.S as -o a.o a.S objdump -S a.o which outputs: a.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <.text>: 0: f4 hlt but it is also documented in the Intel manual of course.

%509s produce 509 spaces. Needed to fill in the file until byte 510.

\125\252 in octal == 0x55 followed by 0xaa . These are 2 required magic bytes which must be bytes 511 and 512. The BIOS goes through all our disks looking for bootable ones, and it only considers bootable those that have those two magic bytes. If not present, the hardware will not treat this as a bootable disk.

If you are not a printf master, you can confirm the contents of main.img with:

hd main.img

which shows the expected:

00000000 f4 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |. | 00000010 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | * 000001f0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 55 aa | U.| 00000200

where 20 is a space in ASCII.

The BIOS firmware reads those 512 bytes from the disk, puts them into memory, and sets the PC to the first byte to start executing them.

Hello world boot sector

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 it for us

VGA: special memory region that gets printed to the screen if written to. Can be used in 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.

serial port. This is a very simple standardized protocol that sends and receives characters from a host terminal. On desktops, it looks like this: Source. It is unfortunately not exposed on most modern laptops, but is the common way to go for development boards, see the ARM examples below. This is really a shame, since such interfaces are really useful to debug the Linux kernel for example.

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 on x86. But note that it is not the most robust method.

main.S

.code16 mov $msg, %si mov $0x0e, %ah loop: lodsb or %al, %al jz halt int $0x10 jmp loop halt: hlt msg: .asciz "hello world"

GitHub upstream.

link.ld

SECTIONS { /* The BIOS loads the code from the disk to this location. * We must tell that to the linker so that it can properly * calculate the addresses of symbols we might jump to. */ . = 0x7c00; .text : { __start = .; *(.text) /* Place the magic boot bytes at the end of the first 512 sector. */ . = 0x1FE; SHORT(0xAA55) } }

Assemble and link with:

as -g -o main.o main.S ld --oformat binary -o main.img -T link.ld main.o qemu-system-x86_64 -hda main.img

Outcome:

And on the T430:

Tested on: Lenovo Thinkpad T430, UEFI BIOS 1.16. Disk generated on an Ubuntu 18.04 host.

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 hlt

int $0x10 : does a BIOS call. This is what prints the characters one by one.

The important link flags are:

--oformat binary : output raw binary assembly code, don't wrap it inside an ELF file as is the case for regular userland executables.

To better understand the linker script part, familiarize yourself with the relocation step of linking: What do linkers do?

Cooler x86 bare metal programs

Here are a few more complex bare metal setups that I've achieved:

Use C instead of assembly

Summary: use GRUB multiboot, which will solve a lot of annoying problems you never thought about. See the section below.

The main difficulty on x86 is that the BIOS only loads 512 bytes from the disk to memory, and you are likely to blow up those 512 bytes when using C!

To solve that, we can use a two-stage bootloader. This makes further BIOS calls, which load more bytes from the disk into memory. Here is a minimal stage 2 assembly example from scratch using the int 0x13 BIOS calls:

Alternatively:

if you only need it to work in QEMU but not real hardware, use the -kernel option, which loads an entire ELF file into memory. Here is an ARM example I've created with that method.

option, which loads an entire ELF file into memory. Here is an ARM example I've created with that method. for the Raspberry Pi, the default firmware takes care of the image loading for us from an ELF file named kernel7.img , much like QEMU -kernel does.

For educational purposes only, here is a one stage minimal C example:

main.c

void main(void) { int i; char s[] = {'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'}; for (i = 0; i < sizeof(s); ++i) { __asm__ ( "int $0x10" : : "a" ((0x0e << 8) | s[i]) ); } while (1) { __asm__ ("hlt"); }; }

entry.S

.code16 .text .global mystart mystart: ljmp $0, $.setcs .setcs: xor %ax, %ax mov %ax, %ds mov %ax, %es mov %ax, %ss mov $__stack_top, %esp cld call main

linker.ld

ENTRY(mystart) SECTIONS { . = 0x7c00; .text : { entry.o(.text) *(.text) *(.data) *(.rodata) __bss_start = .; /* COMMON vs BSS: https://stackoverflow.com/questions/16835716/bss-vs-common-what-goes-where */ *(.bss) *(COMMON) __bss_end = .; } /* https://stackoverflow.com/questions/53584666/why-does-gnu-ld-include-a-section-that-does-not-appear-in-the-linker-script */ .sig : AT(ADDR(.text) + 512 - 2) { SHORT(0xaa55); } /DISCARD/ : { *(.eh_frame) } __stack_bottom = .; . = . + 0x1000; __stack_top = .; }

run

set -eux as -ggdb3 --32 -o entry.o entry.S gcc -c -ggdb3 -m16 -ffreestanding -fno-PIE -nostartfiles -nostdlib -o main.o -std=c99 main.c ld -m elf_i386 -o main.elf -T linker.ld entry.o main.o objcopy -O binary main.elf main.img qemu-system-x86_64 -drive file=main.img,format=raw

C standard library

Things get more fun if you also want to use the C standard library however, since we don't have the Linux kernel, which implements much of the C standard library functionality through POSIX.

A few possibilities, without going to a full-blown OS like Linux, include:

Write your own. It's just a bunch of headers and C files in the end, right? Right??

Newlib Detailed example at: https://electronics.stackexchange.com/questions/223929/c-standard-libraries-on-bare-metal/223931 Newlib implements all the boring non-OS specific things for you, e.g. memcmp , memcpy , etc. Then, it provides some stubs for you to implement the syscalls that you need yourself. For example, we can implement exit() on ARM through semihosting with: void _exit(int status) { __asm__ __volatile__ ("mov r0, #0x18; ldr r1, =#0x20026; svc 0x00123456"); } as shown at in this example. For example, you could redirect printf to the UART or ARM systems, or implement exit() with semihosting.

embedded operating systems like FreeRTOS and Zephyr. Such operating systems typically allow you to turn off pre-emptive scheduling, therefore giving you full control over the runtime of the program. They can be seen as a sort of pre-implemented Newlib.

GNU GRUB Multiboot

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

you have to do a lot of startup yourself, like moving into protected mode

It is for those reasons that GNU GRUB created a more convenient file format called multiboot.

Minimal working example: https://github.com/cirosantilli/x86-bare-metal-examples/tree/d217b180be4220a0b4a453f31275d38e697a99e0/multiboot/hello-world

I also use it on my GitHub examples repo to be able to easily run all examples on real hardware without burning the USB a million times.

QEMU outcome:

T430:

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 /boot .

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 grub-mkrescue .

Firmware

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, -hda says that main.img is a hard disk connected to the hardware, and hda is 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

Registers like %ds and %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.

El Torito

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 isohybrid .

ARM

In ARM, the general ideas are the same.

There is no widely available semi-standardized pre-installed firmware like BIOS for us to use for the IO, so the two simplest types of IO that we can do are:

serial, which is widely available on devboards

blink the LED

I have uploaded:

Some differences from x86 include:

IO is done by writing to magic addresses directly, there is no in and out instructions. This is called memory mapped IO.

for some real hardware, like the Raspberry Pi, you can add the firmware (BIOS) yourself to the disk image. That is a good thing, as it makes updating that firmware more transparent.

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