# multiboot2-elf64
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An experimental Multiboot 2 crate for ELF-64 kernels. It's very incomplete and completely untested. I wrote it for the [blog OS project](https://github.com/phil-opp/blog_os).

Contributions welcome! **If someone wants to maintain this crate, please contact me!**

It uses the Multiboot 1.6 specification at http://nongnu.askapache.com/grub/phcoder/multiboot.pdf and the ELF 64 specification at http://www.uclibc.org/docs/elf-64-gen.pdf.

Below is the draft for a blog post about this. I don't plan to finish it but maybe it's helpful as documentation.

## The Multiboot 2 Information Structure
The Multiboot information structure looks like this:

Field            | Type
---------------- | -----------
total size       | u32
reserved         | u32
tags             | variable
end tag = (0, 8) | (u32, u32)

There are many different types of tags, but they all have the same beginning:

Field         | Type
------------- | -----------------
type          | u32
size          | u32
other fields  | variable

All tags are 8-byte aligned. The last tag must be the _end tag_, which is a tag of type `0` and size `8`.

## Tags

We are interested in two tags, the _Elf-symbols_ tag and the _memory map_ tag. For a full list of possible tags see section 3.4 in the Multiboot 2 specification ([PDF][Multiboot 2]).

[Multiboot 2]: http://nongnu.askapache.com/grub/phcoder/multiboot.pdf

### The Elf-Symbols Tag
The Elf-symbols tag contains a list of all sections of the loaded [ELF] kernel. It has the following format:

[ELF]: http://www.uclibc.org/docs/elf-64-gen.pdf

Field                       | Type
--------------------------- | -----------------
type = 9                    | u32
size                        | u32
number of entries           | u32
entry size                  | u32
string table                | u32
section headers             | variable

Note that this format differs from the description in the Multiboot specification because if seems to be wrong for ELF 64 kernels: The `number of entries`, `entry size`, and `string table` fields seem to be `u32` instead of `u16` (but I'm not sure on this).

The section headers are just copied from the ELF file, so we need to look at the ELF specification to find the corresponding structure definition. Our kernel is a 64-bit ELF file, so we need to look at the ELF-64 specification ([PDF][ELF specification]). According to section 4 and figure 3, a section header has the following format:

[ELF specification]: http://www.uclibc.org/docs/elf-64-gen.pdf

Field                       | Type             | Value
--------------------------- | ---------------- | -----------
name                        | u32              | string table index
type                        | u32              | `0` (unused), `1` (section of program), `3` (string table), `8` (uninitialized section), etc.
flags                       | u64              | `0x1` (writable), `0x2` (loaded), `0x4` (executable), etc.
address                     | u64              | virtual start address of section (0 if not loaded)
file offset                 | u64              | offset (in bytes) of section contents in the file
size                        | u64              | size of the section in bytes
link                        | u32              | associated section (only for some section types)
info                        | u32              | extra information (only for some section types)
address align               | u64              | required alignment of section (power of 2)
entry size                  | u64              | contains the entry size for table sections (e.g. string table)