bits 16
org 0x7c00
-%define ORG 0x7c00
-%define VGA 0xb8000
+; =============================================================================
-%define COLS 80
-%define ROWS 25
+; 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
-%define GRID ORG + 512 ; current cell grid (80 * 25 bytes)
-%define NEXT_GRID GRID + COLS * ROWS ; next cell grid (80 * 25 bytes)
-%define XSS NEXT_GRID + COLS * ROWS ; xs() state
+; constants -------------------------------------------------------------------
+
+%define COLS 80
+%define ROWS 25
+%define VGAPGSZ (COLS * ROWS * 2 + 96) ; why 96 ??????
+
+; 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 es, ax
mov ss, ax
; set stack pointers
-mov sp, ORG
mov bp, ORG
+mov sp, bp
-; clear screen ----------------------------------------------------------------
+; clear direction flag
+cld
; disable cursor
-mov ch, 0x3f
-mov ah, 0x01
-int 0x10
-
-; clear video memory
-mov cx, COLS * ROWS
-mov ax, VGA >> 4
-mov es, ax
-xor di, di
-mov ax, (PRINT_COLOR << 8) | DEAD
-
-rep stosw ; fill cx words at es:di with ax
-
-; setup -----------------------------------------------------------------------
-
-setup:
+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
+mov [xss], dx ; cx:dx = number of clock ticks since midnight
-; initialize grid
-mov di, GRID
-mov cx, COLS * ROWS
+; initialize currvgapg
+mov word [currvgapg], 0
-.initgrid: ; do {
- call xs ; ax = rand
- mov dl, DEAD ; dl = DEAD (likely)
+; start simulation from random state ------------------------------------------
- test ax, 0b11
+start:
- jnz .initgrid_nz ; if (ax % 4 == 0)
- mov dl, ALIVE ; dl = ALIVE
+call init_grid
-.initgrid_nz:
- mov [di + cx], dl ; grid[cx] = dl
+; apply game of life's rules to determine the next state ----------------------
- loop .initgrid ; } while (--cx)
+next_state:
-jmp halt
+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, 0x86
+int 0x15 ; wait
+
+ret
+
+; wait_keypress() - wait for any keypress -------------------------------------
+
+; clobbers: ax
+
+wait_keypress:
+
+mov ah, 0x00 ; read key press
+int 0x16 ; keyboard services
+
+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:
-; ax = updated state ([XSS])
+; [xss] = updated state
-; clobbers ax, dx
+; clobbers bx, dx
xs:
-mov ax, [XSS]
-mov dx, ax
+mov bx, [xss]
+mov dx, bx
shl dx, 1 ; dx = xss << 1
-xor ax, dx ; ax = xss ^ (xss << 1)
-mov dx, ax
+xor bx, dx ; bx = xss ^ (xss << 1)
+mov dx, bx
shr dx, 3 ; dx = xss' >> 3
-xor ax, dx ; ax = xss' ^ (xss' >> 3)
-mov dx, ax
+xor bx, dx ; bx = xss' ^ (xss' >> 3)
+mov dx, bx
shl dx, 10 ; dx = xss'' << 10
-xor ax, dx ; ax = xss'' ^ (xss'' << 10)
-mov [XSS], ax
+xor bx, dx ; bx = xss'' ^ (xss'' << 10)
+mov [xss], bx
ret
-; errors ----------------------------------------------------------------------
+; 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)
-hello:
-mov si, str.hello
+mov di, [currvgapg] ; es:di -> current page
+mov dh, PRINT_COLOR
-; print the error message string in si and halt
-; note: we assume es = VGA_SEG and ds = 0
-printerr:
-xor di, di
-mov ah, PRINT_COLOR
+mov dl, 'P'
+mov [es:di], dx
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+mov dl, al
+add dl, 48
+add di, 2
-write_char:
-lodsb ; al = [ds:si], si += 1
-or al, al ; on null terminator,
-jz halt ; halt
-stosw ; [es:di] = ax, di += 2
-jmp write_char
+mov [es:di], dx
+
+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
+
+ 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
+
+cli ; disable interrupts
hlt
jmp halt
-; data ------------------------------------------------------------------------
-
-str:
-.hello:
- db "lifeboot", 0
+; =============================================================================
times 510 - ($ - $$) db 0 ; fill remaining bytes with zeroes
dw 0xaa55 ; mbr magic byte
projects / lifeboot.git / blobdiff