src/kernel/boot.s

;;; GRUB Multiboot header, calls main() in main.c

	;; Some constants
MBOOT_PAGE_ALIGN    equ 1<<0
MBOOT_MEM_INFO      equ 1<<1
MBOOT_HEADER_MAGIC  equ 0x1BADB002

MBOOT_HEADER_FLAGS  equ MBOOT_PAGE_ALIGN | MBOOT_MEM_INFO
MBOOT_CHECKSUM      equ -(MBOOT_HEADER_MAGIC + MBOOT_HEADER_FLAGS)

	;; Kernel memory start
KERNEL_VIRTUAL_BASE equ 0xC0000000
	;; Index in page directory
KERNEL_PAGE_NUMBER equ (KERNEL_VIRTUAL_BASE >> 22)

STACK_SIZE equ 0x4000



;;;;;;;;;;;;;;;;;;;;; DATA ;;;;;;;;;;;;;;;;;;;;;

	[section .data align = 0x1000]
page_directory:	
	dd 0b010000011				; Identity map first 4 megs
	times (KERNEL_PAGE_NUMBER - 1) dd 0
	dd 0b010000011 				; Map kernel memory to zero page too 
	times (1024 - KERNEL_PAGE_NUMBER - 1) dd 0

gdt:
	;; First entry, null segment
	dw 0 						; zero limit
	dw 0						; base low

	db 0 						; base middle
	db 0						; access
	db 0						; granularity
	db 0						; base high

	;; Second entry, code segment
	dw 0xffff					; max limit
	dw 0

	db 0
	db 0x9a						; access
	db 0xcf 					; granularity
	db 0

	;; Third entry, data segment
	dw 0xffff					; max limit
	dw 0

	db 0
	db 0x92						; access
	db 0xcf 					; granularity
	db 0

	;; Fourth entry, user code segment
	dw 0xffff					; max limit
	dw 0

	db 0
	db 0xfa						; access
	db 0xcf 					; granularity
	db 0

	;; Fifth entry, user data segment
	dw 0xffff					; max limit
	dw 0

	db 0
	db 0xf2						; access
	db 0xcf 					; granularity
	db 0

gdt_pointer:
	dw (8 * 5 - 1)				; sizeof(gdt entry) * 5 - 1
	dd (gdt - KERNEL_VIRTUAL_BASE) ; Remember, PHYSICAL address


;;;;;;;;;;;;;;;;;;;;; CODE ;;;;;;;;;;;;;;;;;;;;;
	
	[bits 32]
	[section .text]
	[global mboot]
	[extern code]
	[extern bss]
	[extern end]
mboot:
	dd  MBOOT_HEADER_MAGIC ; This tells GRUB to start executing here:
	dd  MBOOT_HEADER_FLAGS
	dd  MBOOT_CHECKSUM
   
	dd  mboot                   ; Current location
	dd  code                    ; .text section
	dd  bss                     ; End of .data
	dd  end                     ; End of kernel
	dd  start

	[global start]
	[extern kmain]				; C code

start equ (_start)

_start:
	;; First set up GDT
	mov eax, (gdt_pointer - KERNEL_VIRTUAL_BASE)
	lgdt [eax]					; Load GDT

	mov ax, 0x10				; Offset of data segment
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax

enable_paging:
	mov ecx, (page_directory - KERNEL_VIRTUAL_BASE) ; Physical address
	mov cr3, ecx

	mov ecx, cr4
	or ecx, 0x00000010			; Enable 4 meg pages
	mov cr4, ecx

	mov ecx, cr0
	or ecx, 0x80000000			; Enable paging
	mov cr0, ecx

	lea ecx, [in_higher_half]	; Long jump into high memory
	jmp ecx

in_higher_half:	
	mov dword [page_directory], 0
	invlpg [0]					; Clear identity mapping

	mov esp, (stack + STACK_SIZE)
	
	add ebx, 0xC0000000			; Translate to virtual address
	push ebx					; Holds multiboot header location
	push esp 					; Initial kernel stack
	call kmain

.end:
	hlt
	jmp .end
	

	[section .bss align = 32]
stack:
	resb STACK_SIZE