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 0xfa              ; middle dot
  12 %define FILL_COLOR 0x04             ; red on black
  13 %define PRINT_COLOR 0x07            ; grey on black
  14
  15 ; entry point -----------------------------------------------------------------
  16
  17 ; initialize segment registers
  18 xor ax, ax
  19 mov ds, ax
  20 mov es, ax
  21 mov ss, ax
  22
  23 ; set stack pointers
  24 mov sp, ORG
  25 mov bp, ORG
  26
  27 ; clear screen ----------------------------------------------------------------
  28
  29 ; disable cursor
  30 mov ch, 0x3f
  31 mov ah, 0x01
  32 int 0x10
  33
  34 ; clear video memory
  35 mov cx, VGA_LENW
  36 mov ax, VGA_SEG
  37 mov es, ax
  38 xor di, di
  39 mov ax, (FILL_COLOR << 8) | FILL_CHAR
  40
  41 rep stosw                   ; fill cx words at es:di with ax
  42
  43 ; errors ----------------------------------------------------------------------
  44
  45 hello:
  46 mov si, str.hello
  47
  48 ; print the error message string in si and halt
  49 ; note: we assume es = VGA_SEG and ds = 0
  50 printerr:
  51 xor di, di
  52 mov ah, PRINT_COLOR
  53
  54 ; es:di = video memory
  55 ; ds:si = error message
  56 ; al = current char
  57 ; ah = color attribute
  58
  59 write_char:
  60 lodsb                   ; al = [ds:si], si += 1
  61 or  al, al              ; on null terminator,
  62 jz  halt                ;  halt
  63 stosw                   ; [es:di] = ax, di += 2
  64 jmp write_char
  65
  66 halt:
  67 cli                     ; disable interrupts
  68 hlt
  69 jmp halt
  70
  71 ; data ------------------------------------------------------------------------
  72
  73 str:
  74 .hello:
  75     db "lifeboot", 0
  76
  77 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
  78 dw 0xaa55                   ; mbr magic byte