Thuc notes - OS Kernel Assembly

• 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.

• 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.

checksum:

• Value’s always -(magic number + flags)

• 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.

• 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.

Flags:

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

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

Symbol table:

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

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

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:

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.

• Magic number: 0xE85250D6

3. References

• https://wiki.osdev.org/Bare_Bones
• https://wiki.osdev.org/Multiboot
• https://www.gnu.org/software/grub/manual/multiboot/multiboot.html