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 direction flag
  31 cld
  32
  33 ; disable cursor
  34 mov ch, 0x3f
  35 mov ah, 0x01
  36 int 0x10
  37
  38 ; setup -----------------------------------------------------------------------
  39
  40 setup:
  41
  42 ; initialize xss
  43 mov ah, 0x00                ; get
  44 int 0x1a                    ;  system time
  45 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
  46
  47 ; initialize grid
  48 mov di, GRID                ; es:di -> GRID
  49 mov cx, COLS * ROWS         ; for each cell
  50
  51 .initgrid:                  ; do {
  52     call xs                 ;     [XSS] = rand
  53     mov al, DEAD            ;     al = DEAD (likely)
  54
  55     test [XSS], 0b11
  56
  57     jnz  .initgrid_nz       ;     if ([XSS] % 4 == 0)
  58     mov al, ALIVE           ;         al = ALIVE
  59
  60 .initgrid_nz:
  61     stosb                   ;     [es:(di++)] = al
  62
  63     loop .initgrid          ; } while (--cx)
  64
  65 call print_grid
  66
  67 jmp halt
  68
  69 ; functions -------------------------------------------------------------------
  70
  71 ; xs() - xorshift pseudorandom number generator -------------------------------
  72
  73 ; output:
  74 ; [XSS]     = updated state
  75
  76 ; clobbers bx, dx
  77
  78 xs:
  79
  80 mov bx, [XSS]
  81 mov dx, bx
  82
  83 shl dx, 1                   ; dx = xss << 1
  84 xor bx, dx                  ; bx = xss ^ (xss << 1)
  85 mov dx, bx
  86
  87 shr dx, 3                   ; dx = xss' >> 3
  88 xor bx, dx                  ; bx = xss' ^ (xss' >> 3)
  89 mov dx, bx
  90
  91 shl dx, 10                  ; dx = xss'' << 10
  92 xor bx, dx                  ; bx = xss'' ^ (xss'' << 10)
  93 mov [XSS], bx
  94
  95 ret
  96
  97 ; print_grid() - print GRID cells to screen -----------------------------------
  98
  99 ; (*) this function expects ds = 0 when called and sets es = 0 before
 100 ;     returning. this is simpler because in the rest of the program we want the
 101 ;     segment registers to be zeroed anyway.
 102
 103 ; clobbers ax, cx, di, si
 104
 105 print_grid:
 106
 107 mov ax, VGA >> 4
 108 xor di, di
 109
 110 mov es, ax                  ; es:di -> VGA
 111 mov si, GRID                ; ds:si -> GRID
 112
 113 mov ah, PRINT_COLOR         ; ah = color attribute
 114
 115 mov cx, COLS * ROWS         ; for each cell
 116
 117 .print_cell:                ; do {
 118
 119     lodsb                   ;     al = [ds:(si++)]
 120     stosw                   ;     [es:di] = ax ; di += 2
 121
 122     loop .print_cell        ; } while (--cx)
 123
 124 mov es, cx                  ; es = 0
 125 ret
 126
 127 ; halt() - stop program execution ---------------------------------------------
 128
 129 halt:
 130
 131 cli                         ; disable interrupts
 132 hlt
 133 jmp halt
 134
 135 ; -----------------------------------------------------------------------------
 136
 137 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 138 dw 0xaa55                   ; mbr magic byte