mirror of
https://github.com/phil-opp/blog_os.git
synced 2025-12-17 23:07:50 +00:00
65 lines
2.9 KiB
Markdown
65 lines
2.9 KiB
Markdown
---
|
|
layout: post
|
|
title: '[DRAFT] Rust OS Part 1: Booting'
|
|
related_posts: null
|
|
---
|
|
## Multiboot
|
|
Fortunately there is a bootloader standard: the [Multiboot Specification][multiboot]. So our kernel just needs to indicate that it supports Multiboot and every Multiboot-compliant bootloader can boot it. We will use the [GRUB 2] bootloader together with the Multiboot 2 specification ([PDF][Multiboot 2]).
|
|
|
|
To indicate our Multiboot 2 support to the bootloader, our kernel must contain a _Multiboot Header_, which has the following format:
|
|
|
|
Field | Type | Value
|
|
------------- | --------------- | ----------------------------------------
|
|
magic number | u32 | 0xE85250D6
|
|
architecture | u32 | 0 for i386, 4 for MIPS
|
|
header length | u32 | total header size, including tags
|
|
checksum | u32 | -(magic + architecture + header length)
|
|
tags | variable |
|
|
end tag | (u16, u16, u32) | (0, 0, 8)
|
|
|
|
Converted to a x86 assembly file it looks like this (Intel syntax):
|
|
|
|
```nasm
|
|
section .multiboot_header
|
|
header_start:
|
|
dd 0xe85250d6 ; magic number (multiboot 2)
|
|
dd 0 ; architecture 0 (protected mode i386)
|
|
dd header_end - header_start ; header length
|
|
; checksum
|
|
dd 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))
|
|
|
|
; insert optional multiboot tags here
|
|
|
|
; required end tag
|
|
dw 0 ; type
|
|
dw 0 ; flags
|
|
dd 8 ; size
|
|
header_end:
|
|
```
|
|
If you don't know x86 assembly, here is some quick guide:
|
|
|
|
- the header will be written to a section named `.multiboot_header` (we need this later)
|
|
- `header_start` and `header_end` are _labels_ that mark a memory location. We use them to calculate the header length easily
|
|
- `dd` stands for `define double` (32bit) and `dw` stands for `define word` (16bit)
|
|
- the additional `0x100000000` in the checksum calculation is a small hack[^fn-checksum_hack] to avoid a compiler warning
|
|
|
|
We can already _assemble_ this file (which I called `multiboot_header.asm`) using `nasm`. As it produces a flat binary by default, the resulting file just contains our 24 bytes (in little endian if you work on a x86 machine):
|
|
|
|
```
|
|
> nasm multiboot_header.asm
|
|
> hexdump -x multiboot_header
|
|
0000000 50d6 e852 0000 0000 0018 0000 af12 17ad
|
|
0000010 0000 0000 0008 0000
|
|
0000018
|
|
```
|
|
|
|
|
|
## Booting
|
|
|
|
|
|
[^fn-checksum_hack]: The formula from the table, `-(magic + architecture + header length)`, creates a negative value that doesn't fit into 32bit. By subtracting from `0x100000000` instead, we keep the value positive without changing its truncated value. Without the additional sign bit(s) the result fits into 32bit and the compiler is happy.
|
|
|
|
[multiboot]: https://en.wikipedia.org/wiki/Multiboot_Specification
|
|
[grub 2]: http://wiki.osdev.org/GRUB_2
|
|
[multiboot 2]: http://nongnu.askapache.com/grub/phcoder/multiboot.pdf
|