boot/x86/old/boot.s

   1         bits 16 ; boot sectors run in real mode
   2         org 7C00h ; BIOS loads us at 0x7c00
   3
   4 KERNEL_OFFSET equ 1000h ; where we will load our kernel
   5
   6         mov [BOOT_DRIVE], dl ; BIOS puts the number of our boot drive in dl, so we will want to remember this
   7
   8         mov bp, 9000h ; initialize stack at a spot sufficiently distanced from this code
   9         mov sp, bp
  10
  11         mov di, 0
  12
  13         xor ax, ax ; clear accumulator
  14         int 13h ; BIOS disk services, with a cleared accumulator this will reset the disk controller
  15         jc reset_error ; halt if controller reset fails
  16
  17         mov ah, 2 ; instruct BIOS's interrupt 13h to read sectors
  18         mov al, 30 ; load 30 sectors, might be excessive (for now) but it's still good to do so
  19         xor ch, ch ; read from 1st cylinder
  20         mov cl, 2 ; start reading from 2nd sector, right after the boot sector
  21         xor dh, dh ; read from 1st head
  22         xor bx, bx ; clear B register
  23         mov es, bx ; clear extended segment
  24         mov bx, KERNEL_OFFSET ; put the sectors in our desired offset
  25         ; dl holds the number of the drive to read from, but BIOS already filled this in
  26         int 13h ; do the read
  27         jc read_error ; halt if read fails
  28         cmp al, 10 ; make sure we actually read 10 sectors
  29         jl read_error ; halt if we didn't
  30
  31         call check_a20
  32         cmp ax, 1
  33         jne a20_is_off
  34
  35 a20_is_on
  36         jmp switch_to_pm
  37 a20_is_off
  38         call enable_a20
  39         jmp switch_to_pm
  40
  41 reset_error
  42         mov bx, 0B800h
  43         mov es, bx
  44         mov byte [es:di], '1'
  45         jmp $
  46
  47 read_error
  48         mov bx, 0B800h
  49         mov es, bx
  50         mov byte [es:di], '2'
  51         jmp $
  52
  53 %include "a20.s"
  54 %include "print.s"
  55 %include "protected.s"
  56
  57 BOOT_DRIVE resb 1
  58
  59         times 510-($-$$) db 0
  60         dw 0AA55h ; MBR signature