diff --git a/posts/DRAFT-multiboot-info.md b/posts/DRAFT-multiboot-info.md index 5c0b689c..4cd4d936 100644 --- a/posts/DRAFT-multiboot-info.md +++ b/posts/DRAFT-multiboot-info.md @@ -3,9 +3,74 @@ layout: post title: 'The Multiboot Information Structure' --- -When a multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information struct in the `ebx` register. We can use it to get information about available memory and loaded kernel sections. So let's write a module for it! +When a Multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information structure in the `ebx` register. We can use it to get information about available memory and loaded kernel sections. + +TODO ## The 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`. + +## A Rust module + +TODO + +## 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]: https://en.wikipedia.org/wiki/Executable_and_Linkable_Format + +Field | Type +--------------------------- | ----------------- +type = 9 | u32 +size | u32 +number of entries | u16 +entry size | u16 +string table | u16 +reserved | u16 +section headers | variable + +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) + +### The Memory Map Tag + TODO ## Start and End of Kernel