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let’s boot

Let’s boot some x86 code with qemu.

I rediscovered osdev.org after 13 years of abstinence12 and started off with the plain-assembly babystep. Then I tried the C based bare-bones kernel to have a bit more convenience.

Minimal working example

[ORG 0x7c00]
    jmp hang

times 510-($-$$) db 0
db 0x55
db 0xAA

This nasm code is all you need to make an infinite blinking cursor:

What is going on?

Babystep - plain assembler

After the minimal working example, I approached the plain-assembler babystep bootloader, to get a better insight what’s going on.

Learnings: - basic memory alignment - enter protected mode: code tutorial - write to console without interrupt: code tutorial

Bare-Bones - C

It might be asked much operating at such a low level, but some convenience would be nice. C got you covered! At least a bit. The bare-bones bootloader contains has only as much assembler as necessary to load up C. Basically a stack must be allocated and here we go.

Learnings: - bridge between assembler and C - more sophisticated tty



Bootloaders aren’t just usual3 assembler codes in a hosted environment where an OS is providing all kinds of neat stuff like a libc. There is no such thing as ELF or PE, just the bare-metal CPU which we must satisfy. This not only includes valid assembler code but stuff like:

Memory Layout

In my understanding the assembler file not just contains code and needs to be aligned but is also almost the equivalent of the program that is actual in memory. And because of this, some preprocessor stuff like times 510-($-$$) db 0 has to be made at the end in order it is correctly loaded into the memory.

While researching online about x86 bootloader, two approaches emerged:

It looks like the later approach is mostly used for serious projects, where the former is just a quick n dirty workaround.


Additionally to osdev.org, I also found a bunch of other interesting resources along the way.





  1. Credits to Manawyrm for the inspiration
  2. I already tossed some bits (sorry, german only) 13 years ago but I felt too gatekept and didn’t knew linux.
  3. I couldn’t write that without a laugh. As there would be something like usual assembler…