boot.s

MBOOT_PAGE_ALIGN equ 1 << 0
MBOOT_MEM_INFO equ 1 << 1
MBOOT_USE_GFX equ 0

MBOOT_MAGIC equ 0x1badb002
MBOOT_FLAGS equ MBOOT_PAGE_ALIGN | MBOOT_MEM_INFO | MBOOT_USE_GFX
MBOOT_CHECKSUM equ -(MBOOT_MAGIC + MBOOT_FLAGS)

section .multiboot
ALIGN 4
    DD MBOOT_MAGIC
    DD MBOOT_FLAGS
    DD MBOOT_CHECKSUM
    DD 0, 0, 0, 0, 0

    DD 0
    DD 800
    DD 600
    DD 32

SECTION .bss
ALIGN 16
stack_bottom:
    RESB 16384 * 8
stack_top:

section .boot

global _start
_start:
    MOV ecx, (initial_page_dir - 0xc0000000)
    MOV cr3, ecx

    MOV ecx, cr4
    OR ecx, 0x10
    MOV cr4, ecx

    MOV ecx, cr0
    OR ecx, 0x80000000
    MOV cr0, ecx

    JMP higher_half

section .text
higher_half:
    MOV esp, stack_top
    PUSH ebx
    PUSH eax
    XOR ebp, ebp
    extern kernel_main
    CALL kernel_main

halt:
    hlt
    JMP halt

section .data
align 4096
global initial_page_dir
initial_page_dir:
    DD 10000011b
    TIMES 768-1 DD 0

    DD (0 << 22) | 10000011b
    DD (1 << 22) | 10000011b
    DD (2 << 22) | 10000011b
    DD (3 << 22) | 10000011b
    TIMES 256-4 DD 0