blog_os/_posts/2015-07-22-rust-os-boot.md

layout, title, related_posts

layout	title	related_posts
post	[DRAFT] Rust OS Part 1: Booting	null

Multiboot

Fortunately there is a bootloader standard: the Multiboot Specification. 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).

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):

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 hack¹ 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

---

1. 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. ↩︎