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 PRINT_COLOR 0x07    ; grey on black
  13
  14 %define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
  15 %define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
  16 %define XSS         NEXT_GRID + COLS * ROWS ; xs() state
  17
  18 ; entry point -----------------------------------------------------------------
  19
  20 ; initialize segment registers
  21 xor ax, ax
  22 mov ds, ax
  23 mov es, ax
  24 mov ss, ax
  25
  26 ; set stack pointers
  27 mov sp, ORG
  28 mov bp, ORG
  29
  30 ; clear screen ----------------------------------------------------------------
  31
  32 ; disable cursor
  33 mov ch, 0x3f
  34 mov ah, 0x01
  35 int 0x10
  36
  37 ; clear video memory
  38 mov cx, COLS * ROWS
  39 mov ax, VGA >> 4
  40 mov es, ax
  41 xor di, di
  42 mov ax, (PRINT_COLOR << 8) | DEAD
  43
  44 rep stosw                   ; fill cx words at es:di with ax
  45
  46 ; setup -----------------------------------------------------------------------
  47
  48 setup:
  49
  50 ; initialize xss
  51 mov ah, 0x00                ; get
  52 int 0x1a                    ;  system time
  53 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
  54
  55 ; initialize grid
  56 mov di, GRID
  57 mov cx, COLS * ROWS
  58
  59 .initgrid:                  ; do {
  60     call xs                 ;     ax = rand
  61     mov dl, DEAD            ;     dl = DEAD (likely)
  62
  63     test ax, 0b11
  64
  65     jnz  .initgrid_nz       ;     if (ax % 4 == 0)
  66     mov dl, ALIVE           ;         dl = ALIVE
  67
  68 .initgrid_nz:
  69     mov [di + cx], dl       ;     grid[cx] = dl
  70
  71     loop .initgrid          ; } while (--cx)
  72
  73 jmp halt
  74
  75 ; functions -------------------------------------------------------------------
  76
  77 ; xs() - xorshift pseudorandom number generator -------------------------------
  78
  79 ; output:
  80 ; ax        = updated state ([XSS])
  81
  82 ; clobbers ax, dx
  83
  84 xs:
  85
  86 mov ax, [XSS]
  87 mov dx, ax
  88
  89 shl dx, 1                   ; dx = xss << 1
  90 xor ax, dx                  ; ax = xss ^ (xss << 1)
  91 mov dx, ax
  92
  93 shr dx, 3                   ; dx = xss' >> 3
  94 xor ax, dx                  ; ax = xss' ^ (xss' >> 3)
  95 mov dx, ax
  96
  97 shl dx, 10                  ; dx = xss'' << 10
  98 xor ax, dx                  ; ax = xss'' ^ (xss'' << 10)
  99 mov [XSS], ax
 100
 101 ret
 102
 103 ; errors ----------------------------------------------------------------------
 104
 105 hello:
 106 mov si, str.hello
 107
 108 ; print the error message string in si and halt
 109 ; note: we assume es = VGA_SEG and ds = 0
 110 printerr:
 111 xor di, di
 112 mov ah, PRINT_COLOR
 113
 114 ; es:di = video memory
 115 ; ds:si = error message
 116 ; al = current char
 117 ; ah = color attribute
 118
 119 write_char:
 120 lodsb                   ; al = [ds:si], si += 1
 121 or  al, al              ; on null terminator,
 122 jz  halt                ;  halt
 123 stosw                   ; [es:di] = ax, di += 2
 124 jmp write_char
 125
 126 halt:
 127 cli                     ; disable interrupts
 128 hlt
 129 jmp halt
 130
 131 ; data ------------------------------------------------------------------------
 132
 133 str:
 134 .hello:
 135     db "lifeboot", 0
 136
 137 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 138 dw 0xaa55                   ; mbr magic byte