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
• Babystep - plain assembler
• Bare-Bones - C
• Findings
  • Environment
  • Memory Layout
• Resources
• Vision

Minimal working example

[ORG 0x7c00]
hang:
    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?

• [ORG 0x7c00]: select the right sector, where the bios expects the bootloader
• hang: jmp hang: plain assembly instructing the CPU to infinitely loop by jumping to the jump instruction
• times 510-($-$$) db 0: Bootloader needs to be exactly 512 bytes large in size. This pads the bootloader up with zeroes.
  • pad 510 bytes instead of 512 to leave two bytes free for the magic number
  • $-$$ is a macro calculating <end of file> - <start of file> resulting in the size of the bootloader so far
  • db 0 is just a static byte set to zero
• 0x55aa is the magic number to tell the bios that this is a valid bootloader

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

Findings

Environment

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:

• width: 8, 16 or 32 bit instruction width?
• location: where needs the code to be placed to in order to get (not) executed. The cpu originally doesn’t care whats data and code, it just jumps to a defined address and interprets the corresponding bytes as instruction.

Memory Layout

Resources

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

General:

• Overview about the cpu registers
• BIOS interrupt table
• x86 instruction set
• Instructions

Tutorials:

• Babystep tutorial with pure assembler
• Example bootloader into c
• Bootloader tutorial
• NASM Tutorial
• German tutorial

Special:

• Linux inside handbook
• first linux
• seabios boot which qemu uses per default for booting

Vision

• practice assembly using exercism
• implement a basic shell as entry point for various sub programs