Part of series osdev | 2021-08-29

First part of the kernel. Will be called by the bootloader.
Define a mechanism that the bootloader can recognize and load the system kernel.
It implements the logic that can’t handle from a high-level kernel.
Basic functions:
- Define a function _start that will be jumped to from bootloader as an entry-point.
- Responsible to allocate stack to use.
- Transfer to the high-level kernel by calling to function written by C, C++, Go, Rust etc.
- Keep the computer in an infinite loop.

1. Multiboot standard layout

An interface standard that’s used by the bootloader to recognize and load the system kernel.
Bootloader will search multiboot headers (magic numbers) to recognize the kernel as multiboot compatible.

multiboot header

Offset	Type	Field
0	unsigned 32-bit	magic number
4	unsigned 32-bit	flags
8	unsigned 32-bit	checksum
12	unsigned 32-bit	header_addr
16	unsigned 32-bit	load_addr
20	unsigned 32-bit	load_end_addr
24	unsigned 32-bit	bss_end_addr
28	unsigned 32-bit	entry_addr
32	unsigned 32-bit	mode_type
36	unsigned 32-bit	width
40	unsigned 32-bit	height
44	unsigned 32-bit	depth

1.1.1. Header magic fields

Include: magic number, flags and checksum

Magic number

The hexadecimal value always is 0x1BADB002

Flags:

Control address fields and graphics fields.
Bits 0-15 is a required feature. If the boot loader can’t understand the flag or fulfil them, it fails to load the OS image.
Bits 16-31 is optional features. If the boot loader can’t understand the flag or can’t fulfil them, it’s ignored.

Bit	Description
0	If it’s set, all boot modules loaded with must be aligned on 4KB page boundaries
1	If it’s set, mem_lower and mem_upper in multiboot information structure, that’s passed by the boot loader to OS, must be included. mmap_length and mmap_addr are had in case the boot loader can pass it to.
2	If it’s set, information about video mode table must be available in multiboot information structure, the boot loader passes that to OS, must be included
16	If it’s set, header address fields are used by boot loader instead of the fields in the actual executable header. Don’t need to provide if the kernel image is in ELF format, required if it’s a.out format or in some other format

checksum:

Value’s always -(magic number + flags)

1.1.2. Header address fields

All of the address fields enabled by flag bit 16 are physical addresses.
Include: header_addr, load_addr, load_end_addr, bss_end_addr and entry_addr
header_addr: the physical address of the beginning of the multiboot header.
load_addr: the physical address of the beginning of text segment.
load_end_addr: the physical address of the end of the data segment. So the text and data segment are consecutive in the OS image.
bss_end_addr: the physical address of the end of the BSS segment.
entry_addr: the physical address to jump to start OS from the boot loader.

1.1.3. Header graphics fields

All of the graphics fields are enabled by flag bit 2.
Include: mode_type, width, height and depth
Specify the preferred graphics mode
Recommended mode by OS image, the boot loader may choose a different mode if it sees fit.
mode_type: 0 = linear graphics mode, 1 = EGA-standard text mode
width: The number of columns
height: The number of the lines
depth: The number of bits per pixel in a graphics mode, zero in a text mode.

1.2. Stack definition

Create a section; allocate 16 Kb; create a symbol at the top by setting the esp register. The stack grows downwards on x86.
Aligned 16-byte according to the System V ABI standard and de-facto extensions

1.3. Infinite loop

Disable CLI interrupts
Use hlt to wait for the next interrupt to archive.
Jump back to the waiting step for hlt again if the computer wakes up by any interrupting.

1.4. Multiboot information structure

Multiboot information structure is data that the boot loader communicates to the OS.
The EBX register contains the physical address of the multiboot information structure.

Offset	Size	Field	Description
0	4	flags
4	4	mem_lower	Memory definition
8	4	mem_upper	Memory definition
12	4	boot_device	Boot device information
16	4	cmdline	Command line information
20	4	mods_count	Loadded modules
24	4	mods_addr	Loadded modules
28-40	16	syms	Detail fields between those addresses are controlled by the flag
44	4	mmap_length	Memory map
48	4	mmap_length	Memory map
52	4	drives_length	Drives
56	4	drives_addr	Drives
60	4	config_table	Config table
64	4	boot_loader_name	Boot loader name
68	4	apm_table	Advanced Power Management table
72	16 (2 for each)	vbe_*
88	21	framebuffer_*	framebuffer

Flags:

Indicates features are defined in the following fields will be used.

Bit	Description
0	Enable mem_lower and mem_upper fields to control memory boundary
1	Enable to set the boot_device fields
2	Enable to set the cmdline fields
3	Enable to set the modules fields
4	Defile structure offset 28-40. 4 or 5 is set, another is unset.
6	Enable to set the memory map fields
7	Enable to set the drives fields
8	Enable to set the config table field
9	Enable to set the boot loader name field
10	Enable to set the Advanced Power Management table fields
11	Enable to set the VBE fields
12	Enable to set the framebuffer fields

Memory:

mem_lower, mem_upper.
Define total available memory that can use.

Boot device:

boot_device.
Indicate boot device which BIOSK disk device the boot loader loaded the OS image from
If the OS is not booted by BIOSK disk, it should be clear.

Command line:

cmdline, byte offset = 16, size = 4.
Define the physical address of the command line that is passed to the kernel.

Modules:

mods_count, mods_addr.
Indicate what boot modules were loaded along with the kernel image.
mods_count = number of modules loaded.
mods_addr = physical address of the first module.
Each module will have the following structure

Offset	Size	Field	Description
0	4	mod_start	physical starting address of module
4	4	mod_end	physical ending address of module
8	4	mod_end	arbitrary string of module. It can be command line, pathname
12	4	reserved	set 0 by the boot loader and ignored by OS

Symbol table:

If bit 4 of the flag are set, the following symbol location info is used

Offset	Size	Field	Description
28	4	tabsize	size parameter
32	4	strsize	size parameter
36	4	addr	physical address of and array of a.out format nlist structures
40	4	reserved	just for reversed

If bit 5 of the flag are set, the following s section header table from ELF info is used

Offset	Size	Field	Description
28	4	num	number of entries
32	4	size	size of each entry
36	4	addr	Physical address fields of the ELF section header
40	4	shndx

Memory map:

mmap_length, mmap_addr
Indicate a buffer containing a memory map starting address and its length. BIOS provides the buffer having a memory map. The map provided is guaranteed to list all standard RAM that should be available for usage.
Each item in the memory map has structure:

Offset	Size	Field	Description
-4	4	size	size of memory map item
0	8	base_addr	physical address of the memory that the map define to
8	8	length	the size of the memory region
16	8	type	the variety of address range represented. 1 = available RAM, 3 = usable memory holding ACPI information, 4 = reserved memory which needs to be preserved on hibernation, 5 = occupied by defective RAM modules and all other values currently indicated a reserved area

Drives:

drives_length, drives_addr
Indicate the address of the physical address of the first drive structure and the size of drive structures.

Config table:

config_table
Indicates the address of the ROM configuration table returned by the GET CONFIGURATION BIOS call

Boot loader name:

boot_loader_name
Contains the physical address of the name of a boot loader booting the kernel

Advanced Power Management table:

apm_table
Contains the physical address of an Advanced Power Management table.

VBE

All fields starts with vbe_
Indicate fields of VESA BIOS extensions.

Graphics

All fields starts with framebuffer_
Indicate fields of framebuffer table.

2. Multiboot2 standard layout

TBD.

2.1. Header

Magic number: 0xE85250D6

3. References

OS Kernel Assembly