lifeboot.asm

   1 bits 16
   2 org 0x7c00
   3
   4 %define ORG 0x7c00
   5 %define VGA 0xb8000
   6
   7 %define COLS 80
   8 %define ROWS 25
   9
  10 %define DEAD ' '            ; char to represent a dead cell
  11 %define ALIVE '#'           ; char to represent an alive cell
  12 %define INIT_RATIO 4        ; dead / alive
  13 %define PRINT_COLOR 0x07    ; grey on black
  14
  15 %define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
  16 %define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
  17 %define XSS         NEXT_GRID + COLS * ROWS ; xs() state
  18
  19 ; entry point -----------------------------------------------------------------
  20
  21 ; initialize segment registers
  22 xor ax, ax
  23 mov ds, ax
  24 mov es, ax
  25 mov ss, ax
  26
  27 ; set stack pointers
  28 mov sp, ORG
  29 mov bp, ORG
  30
  31 ; clear screen ----------------------------------------------------------------
  32
  33 ; disable cursor
  34 mov ch, 0x3f
  35 mov ah, 0x01
  36 int 0x10
  37
  38 ; clear video memory
  39 mov cx, COLS * ROWS
  40 mov ax, VGA >> 4
  41 mov es, ax
  42 xor di, di
  43 mov ax, (PRINT_COLOR << 8) | DEAD
  44
  45 rep stosw                   ; fill cx words at es:di with ax
  46
  47 ; setup -----------------------------------------------------------------------
  48
  49 setup:
  50
  51 ; initialize xss
  52 mov ah, 0x00                ; get
  53 int 0x1a                    ;  system time
  54 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
  55
  56 ; functions -------------------------------------------------------------------
  57
  58 ; xs() - xorshift pseudorandom number generator -------------------------------
  59
  60 ; output:
  61 ; ax        = updated state ([XSS])
  62
  63 xs:
  64
  65 mov ax, [XSS]
  66 mov dx, ax
  67
  68 shl dx, 1                   ; dx = xss << 1
  69 xor ax, dx                  ; ax = xss ^ (xss << 1)
  70 mov dx, ax
  71
  72 shr dx, 3                   ; dx = xss' >> 3
  73 xor ax, dx                  ; ax = xss' ^ (xss' >> 3)
  74 mov dx, ax
  75
  76 shl dx, 10                  ; dx = xss'' << 10
  77 xor ax, dx                  ; ax = xss'' ^ (xss'' << 10)
  78 mov [XSS], ax
  79
  80 ret
  81
  82 ; errors ----------------------------------------------------------------------
  83
  84 hello:
  85 mov si, str.hello
  86
  87 ; print the error message string in si and halt
  88 ; note: we assume es = VGA_SEG and ds = 0
  89 printerr:
  90 xor di, di
  91 mov ah, PRINT_COLOR
  92
  93 ; es:di = video memory
  94 ; ds:si = error message
  95 ; al = current char
  96 ; ah = color attribute
  97
  98 write_char:
  99 lodsb                   ; al = [ds:si], si += 1
 100 or  al, al              ; on null terminator,
 101 jz  halt                ;  halt
 102 stosw                   ; [es:di] = ax, di += 2
 103 jmp write_char
 104
 105 halt:
 106 cli                     ; disable interrupts
 107 hlt
 108 jmp halt
 109
 110 ; data ------------------------------------------------------------------------
 111
 112 str:
 113 .hello:
 114     db "lifeboot", 0
 115
 116 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 117 dw 0xaa55                   ; mbr magic byte