bits 16

; calculate logic address of gdt (dx:ax)
mov ax, [cs:gdt_base+0x7c00] ; low 16 bits
mov dx, [cs:gdt_base+0x7c00+2] ; high 16 bits
mov bx, 16
div bx
mov ds, ax ; base
mov bx, dx ; offset

; fill gdt
; first(0) item is always zero (NULL)
; zero based index
mov dword [bx+0x0], 0x0
mov dword [bx+0x4], 0x0
; data seg
mov dword [bx+0x8], 0x8000ffff
mov dword [bx+0xc], 0x0040920b
; code seg
mov dword [bx+0x10], 0x7c0001ff
mov dword [bx+0x14], 0x00409800
; stack seg
mov dword [bx+0x18], 0x00007a00
mov dword [bx+0x1c], 0x00409600

mov word [cs:gdt_bound+0x7c00], 31
lgdt [cs:gdt_bound+0x7c00]

; clear interrupt
cli

; into protect mode
mov eax, cr0
or eax, 1
mov cr0, eax

jmp dword 0x0010:start ; 0000_0000_0001_0000

bits 32

start:
    ; in 32bits mode, set selector for data segment
    mov ax, 0x8 ; 0000_0000_0000_1_000 selectot to second(1) index in GDT
    mov ds, ax

    ; set selecor for stack segment
    mov ax, 0x18 ; 0000_0000_0001_1_000 selector to third(2) index in GDT
    mov ss, ax
    mov esp, 0x7c00

    ; let's show something
    mov eax, 19268
    mov bx, 10
    mov cx, 5
    .loop:
        xor dx, dx
        div bx
        push dx
        loop .loop
    
    mov byte [0], 'e'
    mov byte [1], 0x03 ; cyan on black
    mov byte [2], 'l'
    mov byte [3], 0x03
    mov byte [4], 'd'
    mov byte [5], 0x03
    mov byte [6], 'e'
    mov byte [7], 0x03
    mov byte [8], 'r'
    mov byte [9], 0x03
    mov byte [10], ' '
    mov byte [11], 0x00
    mov cx, 5
    mov bx, 12
    .again:
        pop dx
        add dl, 0x30
        mov [bx], dl
        inc bx
        mov byte [bx], 0x02 ; green on black
        inc bx
        loop .again
    mov byte [bx], ' '
    mov byte [bx+1], 0x00
    mov byte [bx+2], '+'
    mov byte [bx+3], 0x04 ; red on black
    mov byte [bx+4], '1'
    mov byte [bx+5], 0x04
    mov byte [bx+6], 's'
    mov byte [bx+7], 0x04

    hlt    

gdt_bound dw 0
gdt_base dd 0x00007e00

times 510-($-$$) db 0
dw 0xaa55