bits 16 org 0x7c00 ; ============================================================================= ; settings -------------------------------------------------------------------- %define DEAD ' ' ; char to represent a dead cell %define ALIVE '#' ; char to represent an alive cell %define PRINT_COLOR 0x07 ; grey on black %define WAIT_DELAY 0x02 ; 0.131072 seconds ; constants ------------------------------------------------------------------- %define COLS 80 %define ROWS 25 %define VGAPGSZ (COLS * ROWS * 2) ; memory layout --------------------------------------------------------------- %define ORG 0x7c00 %define VGA 0xb8000 %define DAT ORG + 512 ; static variables ------------------------------------------------------------ %define xss DAT ; xs() state (word) %define currvgapg DAT + 2 ; current vga page near pointer (word) ; ============================================================================= ; entry point ----------------------------------------------------------------- ; initialize segment registers mov ax, VGA >> 4 mov es, ax xor ax, ax mov ds, ax mov ss, ax ; set stack pointers mov bp, ORG mov sp, bp ; clear direction flag cld ; disable cursor mov ch, 0x3f ; cursor start and options mov ah, 0x01 ; set text-mode cursor shape int 0x10 ; video services ; initialize xss mov ah, 0x00 ; get int 0x1a ; system time mov [xss], dx ; cx:dx = number of clock ticks since midnight ; initialize currvgapg mov word [currvgapg], VGAPGSZ ; start simulation from random state ------------------------------------------ start: call init_grid ; apply game of life's rules to determine the next state ---------------------- next_state: call flip_vga_page ; display current state call delay ; sleep a little call write_next_vga_page ; write the next state to the hidden page jmp next_state ; ============================================================================= ; functions ------------------------------------------------------------------- ; delay() - suspend program execution temporarily ----------------------------- ; clobbers ah, cx, dx delay: mov cx, WAIT_DELAY ; cx:dx = interval in microseconds mov dx, 0 mov ah, 0x86 int 0x15 ; wait ret ; vsync_wait() - wait for display to enter the next VBlank cycle -------------- ; clobbers ax, dx vsync_wait: mov dx, 0x3da ; input status #1 register .wait_on: in al, dx test al, 0x08 ; vertical retrace bit jnz .wait_on .wait_off: in al, dx test al, 0x08 jz .wait_off ret ; xs() - xorshift pseudorandom number generator ------------------------------- ; output: ; [xss] = updated state ; clobbers bx, dx xs: mov bx, [xss] mov dx, bx shl dx, 1 ; dx = xss << 1 xor bx, dx ; bx = xss ^ (xss << 1) mov dx, bx shr dx, 3 ; dx = xss' >> 3 xor bx, dx ; bx = xss' ^ (xss' >> 3) mov dx, bx shl dx, 10 ; dx = xss'' << 10 xor bx, dx ; bx = xss'' ^ (xss'' << 10) mov [xss], bx ret ; init_grid() - initialize grid cells randomly -------------------------------- ; clobbers ax, bx, cx, dx, di init_grid: mov ah, PRINT_COLOR ; ah = color attribute mov di, [currvgapg] ; es:di -> grid start mov cx, COLS * ROWS ; for each cell .write_cell: ; do { call xs ; [xss] = rand mov al, DEAD ; al = DEAD (likely) test word [xss], 0b11 jnz .nz ; if ([xss] % 4 == 0) mov al, ALIVE ; al = ALIVE .nz: stosw ; [es:di] = ax ; di += 2 loop .write_cell ; } while (--cx) ret ; flip_vga_page() - flip active display page ---------------------------------- ; clobbers ax, dx flip_vga_page: call vsync_wait xor word [currvgapg], VGAPGSZ ; flip currvgapg setnz al ; al = !(currvgapg == 0) mov ah, 0x05 ; select active display page int 0x10 ; video services ret ; alive_neighbours() ---------------------------------------------------------- ; input: ; es:si -> current grid ; cx -> cell index ; output: ; ax = # alive neighbours ; clobbers ax, bx, dx alive_neighbours: enter 5, 0 ; [bp - 1]: row ; [bp - 2]: col ; [bp - 3]: neighbours ; [bp - 4]: i ; [bp - 5]: j mov ax, cx ; ax = idx mov bl, COLS ; bl = COLS div bl ; al = idx / COLS ; ah = idx % COLS mov [bp - 1], al ; row = idx / COLS mov [bp - 2], ah ; row = idx % COLS mov byte [bp - 3], 0 ; neighbours = 0 mov byte [bp - 4], -1 ; i = -1 .i: ; do { mov byte [bp - 5], -1 ; j = -1 .j: ; do { mov al, [bp - 4] or al, [bp - 5] jz .continue ; if (!i && !j) continue mov al, [bp - 1] add al, [bp - 4] ; al = row + i cmp al, 0 jl .continue ; if (row + i < 0) continue cmp al, ROWS jge .continue ; if (row + i >= ROWS) continue mov ah, [bp - 2] add ah, [bp - 5] ; ah = col + j cmp ah, 0 jl .continue ; if (col + j < 0) continue cmp ah, COLS jge .continue ; if (col + j >= COLS) continue movzx bx, ah ; bx = col + j mov ah, COLS mul ah ; ax = (row + i) * COLS add bx, ax ; bx = cell index shl bx, 1 ; bx = grid offset mov dx, [es:si + bx] ; dl = cell state test dl, ALIVE ; if (!ALIVE) continue jne .continue inc byte [bp - 3] ; neighbours++ .continue: inc byte [bp - 5] cmp byte [bp - 5], 1 jle .j ; } while (++j <= 1) inc byte [bp - 4] cmp byte [bp - 4], 1 jle .i ; } while (++i <= 1) movzx ax, byte [bp - 3] ; ax = # alive neighbours leave ret ; write_next_cell_state() ----------------------------------------------------- ; input: ; es:si -> current grid ; es:di -> next grid ; cx -> cell index ; output: ; [es:di + cx * 2] = updated cell state ; clobbers ax, bx, dx write_next_cell_state: call alive_neighbours ; ax = # alive neighbours mov bx, cx shl bx, 1 mov dx, [es:si + bx] ; dl = cell current state test dl, ALIVE ; if (ALIVE) { jne .else test ax, 2 ; if (n < 2) jl .dead ; return DEAD test ax, 3 ; if (n > 3) jg .dead ; return DEAD jmp .alive ; return ALIVE .else: ; } test ax, 3 ; if (n != 3) jne .dead ; return DEAD .alive: ; return ALIVE mov dl, ALIVE jmp .write .dead: mov dl, DEAD .write: mov [es:di + bx], dx ret ; write_next_vga_page() ------------------------------------------------------- ; clobbers di, si, ax, bx, cx, dx write_next_vga_page: mov si, [currvgapg] ; es:si -> current page mov di, VGAPGSZ xor di, si ; es:di -> next page xor cx, cx ; i = 0 .update_loop: call write_next_cell_state inc cx test cx, COLS * ROWS jl .update_loop ; for i in [0 .. COLS * ROWS - 1] ret ; halt() - stop program execution --------------------------------------------- halt: cli ; disable interrupts hlt jmp halt ; ============================================================================= times 510 - ($ - $$) db 0 ; fill remaining bytes with zeroes dw 0xaa55 ; mbr magic byte