bits 16 org 7c00h ; ============================================================================= ; settings -------------------------------------------------------------------- %define DEAD ' ' ; char to represent a dead cell %define ALIVE '#' ; char to represent an alive cell %define PRINT_COLOR 07h ; grey on black %define WAIT_DELAY 02h ; 0.131072 seconds ; constants ------------------------------------------------------------------- %define COLS 80 %define ROWS 25 %define VGAPGSZ (COLS * ROWS * 2 + 96) ; why 96 ?????? ; memory layout --------------------------------------------------------------- %define ORG 7c00h %define VGA 0b8000h %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, 3fh ; cursor start and options mov ah, 01h ; set text-mode cursor shape int 10h ; video services ; initialize xss set_xss_seed: mov ah, 00h ; get int 1ah ; system time cmp dx, 0 je set_xss_seed ; if seed is 0, xs wont work properly mov [xss], dx ; cx:dx = number of clock ticks since midnight ; initialize currvgapg mov word [currvgapg], 0 ; start simulation from random state ------------------------------------------ start: call init_grid ; apply game of life's rules to determine the next state ---------------------- next_state: call vsync_wait call write_next_vga_page call wait_keypress call flip_vga_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, 86h int 15h ; wait ret ; wait_keypress() - wait for any keypress ------------------------------------- ; clobbers: ax wait_keypress: mov ah, 00h ; read key press int 16h ; keyboard services ret ; vsync_wait() - wait for display to enter the next VBlank cycle -------------- ; clobbers ax, dx vsync_wait: mov dx, 3dah ; input status #1 register .wait_on: in al, dx test al, 08h ; vertical retrace bit jnz .wait_on .wait_off: in al, dx test al, 08h 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, 05h ; select active display page int 10h ; 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 cmp 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 cmp dl, ALIVE ; if (ALIVE) { jne .else cmp ax, 2 ; if (n < 2) jl .dead ; return DEAD cmp ax, 3 ; if (n > 3) jg .dead ; return DEAD jmp .alive ; return ALIVE .else: ; } cmp 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 cmp 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 0aa55h ; mbr magic byte