boot.s

   1         bits 16
   2         org 7C00h
   3
   4 KERNEL_OFFSET equ 1000h
   5
   6         mov [BOOT_DRIVE], dl
   7
   8         mov bp, 9000h
   9         mov sp, bp
  10
  11         mov di, 0
  12
  13         xor ax, ax
  14         int 13h
  15         jc reset_error
  16
  17         mov ah, 2
  18         mov al, 10
  19         xor ch, ch
  20         mov cl, 2
  21         xor dh, dh
  22         xor bx, bx
  23         mov es, bx
  24         mov bx, KERNEL_OFFSET
  25         int 13h
  26         jc read_error
  27         cmp al, 10
  28         jl read_error
  29         jmp switch_to_pm
  30
  31 reset_error
  32         mov bx, 0B800h
  33         mov es, bx
  34         mov byte [es:di], '1'
  35         jmp halt
  36
  37 read_error
  38         mov bx, 0B800h
  39         mov es, bx
  40         mov byte [es:di], '2'
  41         jmp halt
  42
  43         call switch_to_pm
  44
  45 halt
  46         jmp $
  47
  48 switch_to_pm
  49         mov bx, 0B800h
  50         mov es, bx
  51         mov byte [es:0], 'L'
  52
  53         cli
  54         xor ax, ax
  55         mov ds, ax
  56         lgdt [gdt_desc]
  57
  58         mov eax, cr0
  59         or eax, 1
  60         mov cr0, eax
  61         jmp CODE_SEG:protected
  62
  63         bits 32
  64 protected
  65         mov ax, DATA_SEG
  66         mov ds, ax
  67         mov ss, ax
  68         mov es, ax
  69         mov fs, ax
  70         mov gs, ax
  71
  72         mov ebp, 090000h
  73         mov esp, ebp
  74
  75         mov ebx, pm_success
  76         call pmprint
  77
  78         call KERNEL_OFFSET
  79
  80         jmp $
  81
  82 pmprint
  83         pusha
  84         mov edx, video_memory
  85
  86 .loop
  87         mov al, [ebx]
  88         mov ah, 0Fh
  89
  90         cmp al, 0
  91         je .done
  92
  93         mov [edx], al
  94         mov [edx+1], ah
  95
  96         inc ebx
  97         add edx, 2
  98
  99         jmp .loop
 100
 101 .done
 102         popa
 103         ret
 104
 105 pm_success db "Now in protected mode", 0
 106 video_memory equ 0B8000h
 107
 108 gdt_start
 109 null_seg
 110         dq 0
 111 code_seg
 112         dw 0FFFFh
 113         dw 0
 114         db 0
 115         db 10011010b
 116         db 11001111b
 117         db 0
 118 data_seg
 119         dw 0FFFFh
 120         dw 0
 121         db 0
 122         db 10010010b
 123         db 11001111b
 124         db 0
 125 gdt_end
 126 gdt_desc
 127         dw gdt_end - gdt_start - 1
 128         dd gdt_start
 129 CODE_SEG equ code_seg - gdt_start
 130 DATA_SEG equ data_seg - gdt_start
 131
 132 BOOT_DRIVE db 0
 133
 134         times 510-($-$$) db 0
 135         dw 0AA55h