global start
global heap_start
extern kernel_main
bits 32

; base, limit, access, flags
%macro gdt_entry 4
	db %2 & 0xff
	db (%2 >> 8) & 0xff
	db %1 & 0xff
	db (%1 >> 8) & 0xff
	db (%1 >> 16) & 0xff
	db %3
	db ((%2 >> 16) & 0x0f) | (%4 << 4)
	db (%1 >> 24) & 0xff
%endmacro

MAGIC    equ  0xe85250d6
LENGTH   equ  mb_end - mb_start
CHECKSUM equ  -(MAGIC + LENGTH)
 
section .multiboot
align 8
mb_start:
dd MAGIC
dd 0
dd LENGTH
dd CHECKSUM
dw 0
dw 0
dd 8
mb_end:
 
section .bss
align 16
stack_end:
resb 16384
stack_top:
 
section .rodata
align 16
gdt_start:
gdt_entry 0, 0, 0, 0
gdt_entry 0, 0xFFFFF, 0x9A, 0xC
gdt_entry 0, 0xFFFFF, 0x92, 0xC
gdt_end:
gdt_descriptor:
	dw gdt_end - gdt_start - 1
	dd gdt_start

section .text
align 8
start:
	cli
	lgdt [gdt_descriptor]
    mov eax, cr0
    or al, 1
    mov cr0, eax
	jmp 0x08:after_lgdt
after_lgdt:
	mov ax, 0x10
	mov ds, ax
	mov ss, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov esp, stack_end
	mov ebp, stack_end
	sti
	push ebx
	call kernel_main
	cli
halt:
	hlt
	jmp halt