Goal: given a hard drive and some way of writing bytes to that hard drive, how could I run code on a computer that doesn't have an operating system installed.
I want it to be clear that what happens after the computer boots is completely dependent on the hardware.
The common mechanism is that there's some code stored on your machine that is loaded and executed immediately when the machine is powered on, and this code "searches" for more code to load and execute which is (hopefully) stored on a hard drive, floppy disk, cd, etc.
In my opinion, the implementation details of this are worth digging into for at least one architecture type. Here, I'm going to walk you through the implementation details of two different modes: BIOS (for x86) and UEFI (most modern CPUs).
Why do I think this is worth spending your time on?
- You'll very quickly appreciate the fact that bare-metal programming is very different to what you're used to
- You'll spend a good amount of time with gdb, which is the number one most useful tool in OS dev
- You'll develop a deep understanding of the very early booting stages of an OS which provides a strong foundation for understanding higher-level OS concepts
BIOS (qemu-system-i386)
Computer powers on and BIOS loads the first sector (512 bytes) of hda into 0x7c00.
We can write instructions to this first sector, and they will be executed by the CPU.
Problem: a sector is only 512 bytes, so we can't write very sophisticated programs here.
Problem: real mode (?)
Some things I want to consider in this:
- Bootloader: getting a hello world kernel
- Interrupts
- Internal memory management: kernel malloc/free
- Running code at different privilege levels (?)
- File-system support (?)