bootloader.asm

f.asm

use16
org 0x7c00

    ; Set stack
    mov bp, 0x8000
    mov sp, bp

    ; Load 2nd stage
    mov bx, 0x07E0
    mov es, bx
    mov ds, bx
    xor bx, bx

    mov al, 0x01    ; number of sectors to read
    mov ch, 0x00    ; cylinder number
    mov cl, 0x02    ; sector number
    mov dh, 0x00    ; head number
    mov dl, 0x80    ; drive number (using a disk drive in qemu)
    
    clc
    mov ah, 0x02
    int 0x13
    jc error        ; error handling is omitted, not important & doesn't cause problems

    jmp 0x07E0:0x00
    
times 510 - ($-$$) db 0
dw 0xaa55

    
    
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; second stage code
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

use16
org 0x7E00

bootsect_second_stage:
    ; WITHOUT THIS BIT IT DOESN'T WORK...
    ; WHY DO I NEED TO SET THE SEGMENT REGISTERS to 0... (and not 0x07E0 for example)?
    mov bx, 0
    mov ds, bx
    mov es, bx
    mov fs, bx
    mov gs, bx
    mov ss, bx

    call switch_to_protected_mode

    cli
    hlt

gdt_start:
gdd_null:
    dq 0
gdt_code:
    dw 0xFFFF
    dw 0
    db 0
    db 10011010b
    db 11001111b
    db 0
gdt_data:
    dw 0xFFFF
    dw 0
    db 0
    db 10010010b
    db 11001111b
    db 0
gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1
    dd gdt_start

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

switch_to_protected_mode:
    mov bx, switching_to_protected_mode
    call print_str

    cli
    lgdt [gdt_descriptor]
    mov eax, cr0
    or eax, 1
    mov cr0, eax

    jmp CODE_SEG:protected_mode_start
    ret ; Should never be reached

use32
protected_mode_start:
    mov ax, DATA_SEG
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    mov esp, 090000h

    ; Write 'A' to the middle of the screen (beginning of the video memory is at 0xB8000)
    mov edi, 0xB85EC
    mov byte [edi], 'A'
    mov edi, 0xB85ED
    mov byte [edi], 4

    hlt

times 512 - ($ - $$) db 0