Question

I ve already done a part of my OS in Assembly, but now I want to build a own bootloader for it too instead of using GRUB. When I was developing my test OS in Assembly I remember that I boot it like this:

org 0x7c00
bits 16

; OS Kernel Here

times 510 - ($-$$) db 0
dw 0xAA55

This I ve already know. Now I want to use this and execute the "real" OS that will be a *.bin file written to the 2nd sector of the floppy. Then I want to know somethings

How can I do a bootloader in Assembly to execute what will be starting on the 2nd sector of the floppy?
I need to add anything to the Assembly source that will be placed on the 2nd sector of the floppy?

Answer 1

You use int 0x13 to load in the required number of sectors and jump to the location you have place the new code. There is nothing you need to do in the second stage, but you will want to make sure that you set DS to be valid for wherever you load the code.

Example from my little OS archive:

  /* BIOS loads the sectors into es:bx */
  pushw    $STAGE1_WORKSEG
  popw     %es
  movw     $STAGE1_OFFSET, %bx

read_stage1:

  /* Try to read in a few sectors */
  movb     $0x2, %cl      /* Sector */
  movb     $0x0, %ch      /* Cylinder */
  movb     $0x0, %dh      /* Head */
  movb     $0x0, %dl      /* Drive */
  movb     $0x2, %ah      /* BIOS read function */

  /* How many sectors to load */
  movb     $STAGE1_SIZE, %al 
  int      $0x13
  jnc      read_stage1_done

  /* Reset drive */
  xorw     %ax, %ax
  int      $0x13
  jmp      read_stage1


read_stage1_done:

  /* Perform a long jump into stage1 */
  ljmp     $STAGE1_WORKSEG, $STAGE1_OFFSET

  call     halt

halt:
    /*
    * Function: halt
    * Synopsis: Sends the processor into a permanent halted status
    * Notes:
    *    The only way out of this is to manually reboot
    */
    hlt                     /* Halt the processor */
    jmp      halt

That s in GAS format so you ll want to reverse the operand order because it looks like you re using NASM from the times instruction. The variable names should be self-explanatory.

If you re developing a hobby OS then http://forum.osdev.org/ is a good place to get support from others doing the same thing. It s a bit more specialised than stackoverflow and a lot of OS stuff can be quite esoteric.

Answer 2

While in 16-bit mode you can use the code below to load your kernel from disk into memory:

disk_load:
    push dx

    mov ah, 0x02
    mov al, dh
    mov ch, 0x00
    mov dh, 0x00
    mov cl, 0x02

    int 0x13

    jc disk_error

    pop dx
    cmp dh, al
    jne disk_error
    ret

disk_error:

    mov bx, DISK_ERROR_MSG
    call print_string
    jmp $

DISK_ERROR_MSG db "Disk read error !", 0

    load_kernel:
        mov bx, MSG_LOAD_KERNEL
        call print_string
        mov bx, KERNEL_OFFSET
        mov dh, 54
        mov dl, [BOOT_DRIVE]
        call disk_load
        ret

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