lifeboot.asm

   1 bits 16
   2 org 0x7c00
   3
   4 %define ORG 0x7c00                  ; where we are loaded initially
   5
   6 %define VGA_SEG 0xb800              ; video memory starts at 0xb8000
   7 %define VGA_COL 80
   8 %define VGA_ROW 25
   9 %define VGA_LENW VGA_COL * VGA_ROW
  10
  11 %define FILL_CHAR ' '
  12 %define PRINT_COLOR 0x07            ; grey on black
  13
  14 ; entry point -----------------------------------------------------------------
  15
  16 ; initialize segment registers
  17 xor ax, ax
  18 mov ds, ax
  19 mov es, ax
  20 mov ss, ax
  21
  22 ; set stack pointers
  23 mov sp, ORG
  24 mov bp, ORG
  25
  26 ; clear screen ----------------------------------------------------------------
  27
  28 ; disable cursor
  29 mov ch, 0x3f
  30 mov ah, 0x01
  31 int 0x10
  32
  33 ; clear video memory
  34 mov cx, VGA_LENW
  35 mov ax, VGA_SEG
  36 mov es, ax
  37 xor di, di
  38 mov ax, (PRINT_COLOR << 8) | FILL_CHAR
  39
  40 rep stosw                   ; fill cx words at es:di with ax
  41
  42 ; errors ----------------------------------------------------------------------
  43
  44 hello:
  45 mov si, str.hello
  46
  47 ; print the error message string in si and halt
  48 ; note: we assume es = VGA_SEG and ds = 0
  49 printerr:
  50 xor di, di
  51 mov ah, PRINT_COLOR
  52
  53 ; es:di = video memory
  54 ; ds:si = error message
  55 ; al = current char
  56 ; ah = color attribute
  57
  58 write_char:
  59 lodsb                   ; al = [ds:si], si += 1
  60 or  al, al              ; on null terminator,
  61 jz  halt                ;  halt
  62 stosw                   ; [es:di] = ax, di += 2
  63 jmp write_char
  64
  65 halt:
  66 cli                     ; disable interrupts
  67 hlt
  68 jmp halt
  69
  70 ; data ------------------------------------------------------------------------
  71
  72 str:
  73 .hello:
  74     db "lifeboot", 0
  75
  76 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
  77 dw 0xaa55                   ; mbr magic byte