%define DEAD ' ' ; char to represent a dead cell
%define ALIVE '.' ; char to represent an alive cell
%define PRINT_COLOR 07h ; grey on black
-%define ITER_LIM 500 ; reset the simulation after ITER_LIM iterations
+%define ITER_LIM 500 ; reset the simulation after this # iterations
; constants -------------------------------------------------------------------
%define COLS 80
%define ROWS 25
-%define VGAPGSZ 1000h
+%define VGAPGSZ 1000h ; (bytes)
; memory layout ---------------------------------------------------------------
; entry point -----------------------------------------------------------------
+entry:
+
; initialize segment registers
mov ax, VGA >> 4
-mov es, ax
+mov es, ax ; es:0 -> video memory
xor ax, ax
-mov ds, ax
-mov ss, ax
+mov ds, ax ; ds = 0
+mov ss, ax ; ss = 0
; set stack pointers
mov bp, ORG
; disable cursor
mov ch, 3fh ; cursor start and options
mov ah, 01h ; set text-mode cursor shape
-int 10h ; video services
+int 10h ; bios video services
; initialize xss
+.set_xss:
+ mov ah, 00h ; get system time
+ int 1ah ; bios time services
+ ; cx:dx = # clock ticks since midnight
+ or dx, dx
+ jz .set_xss ; wait for something meaningful
-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
+mov [xss], dx
; initialize currvgapg
mov word [currvgapg], 0
start:
call init_grid
-mov word [iter], 1
-
-; apply game of life's rules to determine the next state ----------------------
+mov word [iter], 0
-next_state:
+.next_state:
-call vsync_wait
-call write_next_vga_page
+ call vsync_wait
+ call update_grid
-call vsync_wait
-call flip_vga_page
+ call vsync_wait
+ call flip_vgapg
-inc word [iter]
-cmp word [iter], ITER_LIM
-jle next_state
+ inc word [iter]
+ cmp word [iter], ITER_LIM
+ jl .next_state
jmp start ; reset
; functions -------------------------------------------------------------------
-; vsync_wait() - wait for display to enter the next VBlank cycle --------------
+; vsync_wait() - wait for display to enter the next retrace cycle -------------
-; clobbers ax, dx
+; clobbers al, dx
vsync_wait:
mov dx, 3dah ; input status #1 register
-.wait_on:
+.wait_retrace_end:
in al, dx
test al, 08h ; vertical retrace bit
- jnz .wait_on
+ jnz .wait_retrace_end
-.wait_off:
+.wait_retrace_start:
in al, dx
test al, 08h
- jz .wait_off
+ jz .wait_retrace_start
ret
; output:
; [xss] = updated state
+; bx = [xss]
; clobbers bx, dx
mov cx, COLS * ROWS ; for each cell
.write_cell: ; do {
- call xs ; [xss] = rand
- mov al, DEAD ; al = DEAD (likely)
+ call xs ; bx = random value
+ mov al, DEAD ; al = DEAD
- test word [xss], 0b11
+ test bx, 0b11
- jnz .nz ; if ([xss] % 4 == 0)
+ jnz .nz ; if (bx % 4 == 0)
mov al, ALIVE ; al = ALIVE
.nz:
- stosw ; [es:di] = ax ; di += 2
+ stosw ; [es:di] = ax, di += 2
loop .write_cell ; } while (--cx)
ret
-; flip_vga_page() - flip active display page ----------------------------------
+; flip_vgapg() - flip active display page -------------------------------------
-; clobbers ax, dx
+; output:
+; [currvgapg] ^= VGAPGSZ
-flip_vga_page:
+; clobbers ax
-xor word [currvgapg], VGAPGSZ ; flip currvgapg
-setnz al ; al = !(currvgapg == 0)
+flip_vgapg:
+xor word [currvgapg], VGAPGSZ
+
+setnz al ; al = !(currvgapg == 0)
mov ah, 05h ; select active display page
-int 10h ; video services
+int 10h ; bios video services
ret
-; alive_neighbours() ----------------------------------------------------------
+; alive_neighbours() - get number of alive adjacent cells ---------------------
; input:
; es:si -> current grid
mov ax, cx ; ax = idx
mov bl, COLS ; bl = COLS
-div bl ; al = idx / COLS ; ah = idx % COLS
+div bl ; al = idx / COLS, ah = idx % COLS
mov [bp - 1], al ; row = idx / COLS
mov [bp - 2], ah ; row = idx % COLS
.j: ; do {
mov al, [bp - 4]
- or al, [bp - 5]
- jz .continue ; if (!i && !j) continue
+ or al, [bp - 5] ; if (!i && !j)
+ jz .continue ; 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
+ cmp al, ROWS ; if ((unsigned)(row + i) >= ROWS)
+ jae .continue ; 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
+ cmp ah, COLS ; if ((unsigned)(col + j) >= COLS)
+ jae .continue ; continue
movzx bx, ah ; bx = col + j
mov dx, [es:si + bx] ; dl = cell state
- cmp dl, ALIVE ; if (!ALIVE) continue
- jne .continue
+ cmp dl, ALIVE ; if (!ALIVE)
+ jne .continue ; continue
inc byte [bp - 3] ; neighbours++
leave
ret
-; write_next_cell_state() -----------------------------------------------------
+; update_cell() - write next cell state ---------------------------------------
; input:
; es:si -> current grid
; clobbers ax, bx, dx
-write_next_cell_state:
+update_cell:
call alive_neighbours ; ax = # alive neighbours
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
+shr ax, 1
+cmp ax, 1 ; if (n != 2 || n != 3)
+jne .dead ; return DEAD
jmp .alive ; return ALIVE
ret
-; write_next_vga_page() -------------------------------------------------------
+; update_grid() - write next grid (inactive vga page) -------------------------
; clobbers di, si, ax, bx, cx, dx
-write_next_vga_page:
+update_grid:
mov si, [currvgapg] ; es:si -> current page
mov di, VGAPGSZ
xor di, si ; es:di -> next page
-xor cx, cx ; i = 0
+mov cx, COLS * ROWS - 1 ; for each cell (index)
+
+.update_loop: ; do {
-.update_loop:
-call write_next_cell_state
+ call update_cell
-inc cx
-cmp cx, COLS * ROWS
-jl .update_loop ; for i in [0 .. COLS * ROWS - 1]
+ dec cx
+ jns .update_loop ; } while (--cx > 0)
ret
projects / lifeboot.git / blobdiff