Bare Bones Kernel

Preface

This tutorial assumes you have a compiler / assembler / linker toolchain capable of handling ELF files. On a Windows machine, you are strongly encouraged to set up a GCC Cross-Compiler, as it removes all the various toolchain-specific issues you might have ("PE operation on a non-PE file", "unsupported file format", and a number of others). While technically a Linux machine already has an ELF-capable toolchain, you are still encouraged to build a cross-compiler, as it is the first step to do, and it keeps you from relying on things you shouldn't (header files, for example).

The code in this tutorial assumes you are using GRUB to boot the system. GRUB (a Multiboot compliant boot loader) puts the system in to the correct state for your kernel to start executing. This includes enabling the A20 line (to give you access to all available memory addresses) and putting the system in to 32-bit Protected Mode, giving you access to a theoretical 4GiB of memory.

Overview

Even when using GRUB, some setup is required before entering an int main() type function. The most basic setup to get an ELF format kernel to be booted by GRUB consists of three files:

loader.s - assembler "glue" between bootloader and kernel

kernel.c - your actual kernel routines

linker.ld - for linking the above files

The second part of this tutorial briefly describes how to boot the compiled kernel.

loader.s

loader.s takes over over control from the Multiboot bootloader, and jumps into the kernel proper.

NASM

global loader           ; making entry point visible to linker

extern kmain            ; kmain is defined elsewhere

 

; setting up the Multiboot header - see GRUB docs for details

MODULEALIGN equ  1<<0                   ; align loaded modules on page boundaries

MEMINFO     equ  1<<1                   ; provide memory map

FLAGS       equ  MODULEALIGN | MEMINFO  ; this is the Multiboot 'flag' field

MAGIC       equ    0x1BADB002           ; 'magic number' lets bootloader find the header

CHECKSUM    equ -(MAGIC + FLAGS)        ; checksum required

 

section .text

align 4

MultiBootHeader:

   dd MAGIC

   dd FLAGS

   dd CHECKSUM

 

; reserve initial kernel stack space

STACKSIZE equ 0x4000                  ; that's 16k.

 

loader:

   mov esp, stack+STACKSIZE           ; set up the stack

   push eax                           ; pass Multiboot magic number

   push ebx                           ; pass Multiboot info structure

 

   call  kmain                       ; call kernel proper

...

Read more: OsDev.org wiki

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