mbr_asm/mbr.asm

section .boot
bits 16
global boot
boot:
	jmp main

display_enable:
    push bp
    mov bp, sp
    mov ah, 0h      ; 00h    Set Video Mode
    mov al, 07h     ; Txt, monochrome, 80x25

    int 10h

    mov sp, bp
    pop bp
    ret

print:
    push bp
    mov bp, sp
    mov si, [bp + 4]; put first function arg in si. sp is stask pointer
.loop:
    lodsb           ; load si content into al then inc si
    cmp al, 0;
    je .end

    mov ah, 0eh
    mov bx, 0
    int 10h

    jmp .loop

.end:
    mov sp, bp
    pop bp
    ret

println:
    push bp
    mov bp, sp
    push word [bp + 4]
    call print
    add sp, 2

    mov ah, 03h     ; read cursor position
    int 10h         ; row number in dh. Col in dl

    inc dh          ; goto next line
    mov dl, 0

    mov ah, 02h     ; Set Cursor Position
    int 10h
    mov sp, bp
    pop bp
    ret

hello db 'Booting matOs', 0

main:
    sti             ; enable virtual interupts
    mov [disk],dl   ; save disk used to boot by bios

    call display_enable

    push hello
    call println
    add sp, 2

; Switch in 32bits Protected mode

    ; Activate A20 http://wiki.osdev.org/A20_Line to be able to access more than 1Mb memory
    mov ah, 0h
    mov ax, 0x2401
    int 0x15

    ; Change video mode to display VGA
    mov ax, 0x3
    int 0x10

    ; http://www.ctyme.com/intr/rb-0607.htm
    ; Bios read first 512 bytes, read next disk sector
    mov ah, 0x2    ;read sectors
    mov al, 10     ;sectors to read
    mov ch, 0      ;cylinder idx
    mov dh, 0      ;head idx
    mov cl, 2      ;sector idx
    mov dl, [disk] ;disk idx
    mov bx, copy_target;target pointer
    int 0x13

    cli ; disable interruption when setting GDT

    ; switch in 32 bits
    lgdt [gdt_pointer] ; switch in 32bits here
    mov eax, cr0
    or eax,0x1; set the protected mode bit on special CPU reg cr0
    mov cr0, eax
    jmp CODE_SEG:boot2 ; In protected mode we need to add the segment selector

; GDT table desciption could be found http://wiki.osdev.org/Global_Descriptor_Table
; here we define the 3 64bits segment needed: null segment, code segment and data segment
gdt_start: ;null segment
	dq 0x0
gdt_code:  ;code segment
	dw 0xFFFF	; limit [0:15]
	dw 0x0		; base [0:15]
	db 0x0		; base [16:23]
	db 10011010b    ; access byte:  Present(1)| Priv(2) 0 ->kernel 3->userspace | 1 | Executable(1) | Direction/Conformity (1) | RW(1) | Accessed(1)
	db 11001111b	; Granularity(1) | Size (1) 0-> 16bit mode 1->32protected mode | 0 | 0 | Limit [16:19]
	db 0x0		; base [24:31]
gdt_data:
	dw 0xFFFF
	dw 0x0
	db 0x0
	db 10010010b
	db 11001111b
	db 0x0
gdt_end:
gdt_pointer:
	dw gdt_end - gdt_start
	dd gdt_start
disk:
	db 0x0
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

times 510 - ($-$$) db 0
dw 0xaa55
copy_target:
bits 32
boot2:
	mov ax, DATA_SEG ; set all segments to point to DATA_SEG https://en.wikipedia.org/wiki/X86_memory_segmentation
	mov ds, ax       ; Data segment
	mov es, ax	 ; Extra Segment (for string operation)
	mov fs, ax	 ; No Specific use
	mov gs, ax	 ; No Specific use
	mov ss, ax	 ; stack segment
	mov esi,hello32
	mov ebx,0xb8000 ; Cannot use BIOS anymore, use VGA Text buffer instead

.loop32:
	lodsb
	or al,al
	jz halt
	or eax,0x0100 ; blue bg
	mov word [ebx], ax
	add ebx,2
	jmp .loop32
halt:
	mov esp,kernel_stack_top
	extern kmain
	call kmain
	cli
	hlt
hello32: db "Hello 32 bits world!",0
section .bss
align 4
kernel_stack_bottom: equ $
	resb 16384 ; 16 KB
kernel_stack_top: