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 ; clear screen ----------------------------------------------------------------
  34
  35 ; disable cursor
  36 mov ch, 0x3f
  37 mov ah, 0x01
  38 int 0x10
  39
  40 ; clear video memory
  41 mov cx, COLS * ROWS
  42 mov ax, VGA >> 4
  43 mov es, ax
  44 xor di, di
  45 mov ax, (PRINT_COLOR << 8) | DEAD
  46
  47 rep stosw                   ; fill cx words at es:di with ax
  48
  49 ; setup -----------------------------------------------------------------------
  50
  51 setup:
  52
  53 ; initialize xss
  54 mov ah, 0x00                ; get
  55 int 0x1a                    ;  system time
  56 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
  57
  58 ; initialize grid
  59 mov cx, COLS * ROWS         ; for each cell
  60 xor ax, ax
  61 mov es, ax
  62 mov di, GRID                ; es:di -> GRID
  63
  64 .initgrid:                  ; do {
  65     call xs                 ;     [XSS] = rand
  66     mov al, DEAD            ;     al = DEAD (likely)
  67
  68     test [XSS], 0b11
  69
  70     jnz  .initgrid_nz       ;     if ([XSS] % 4 == 0)
  71     mov al, ALIVE           ;         al = ALIVE
  72
  73 .initgrid_nz:
  74     stosb                   ;     [es:(di++)] = al
  75
  76     loop .initgrid          ; } while (--cx)
  77
  78 jmp halt
  79
  80 ; functions -------------------------------------------------------------------
  81
  82 ; xs() - xorshift pseudorandom number generator -------------------------------
  83
  84 ; output:
  85 ; [XSS]     = updated state
  86
  87 ; clobbers bx, dx
  88
  89 xs:
  90
  91 mov bx, [XSS]
  92 mov dx, bx
  93
  94 shl dx, 1                   ; dx = xss << 1
  95 xor bx, dx                  ; bx = xss ^ (xss << 1)
  96 mov dx, bx
  97
  98 shr dx, 3                   ; dx = xss' >> 3
  99 xor bx, dx                  ; bx = xss' ^ (xss' >> 3)
 100 mov dx, bx
 101
 102 shl dx, 10                  ; dx = xss'' << 10
 103 xor bx, dx                  ; bx = xss'' ^ (xss'' << 10)
 104 mov [XSS], bx
 105
 106 ret
 107
 108 ; errors ----------------------------------------------------------------------
 109
 110 hello:
 111 mov si, str.hello
 112
 113 ; print the error message string in si and halt
 114 ; note: we assume es = VGA_SEG and ds = 0
 115 printerr:
 116 xor di, di
 117 mov ah, PRINT_COLOR
 118
 119 ; es:di = video memory
 120 ; ds:si = error message
 121 ; al = current char
 122 ; ah = color attribute
 123
 124 write_char:
 125 lodsb                   ; al = [ds:si], si += 1
 126 or  al, al              ; on null terminator,
 127 jz  halt                ;  halt
 128 stosw                   ; [es:di] = ax, di += 2
 129 jmp write_char
 130
 131 halt:
 132 cli                     ; disable interrupts
 133 hlt
 134 jmp halt
 135
 136 ; data ------------------------------------------------------------------------
 137
 138 str:
 139 .hello:
 140     db "lifeboot", 0
 141
 142 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 143 dw 0xaa55                   ; mbr magic byte