bits 16
org 0x7c00
-%define ORG 0x7c00 ; where we are loaded initially
+; =============================================================================
-%define VGA_SEG 0xb800 ; video memory starts at 0xb8000
-%define VGA_COL 80
-%define VGA_ROW 25
-%define VGA_LENW VGA_COL * VGA_ROW
+; settings --------------------------------------------------------------------
-%define FILL_CHAR 0xfa ; middle dot
-%define FILL_COLOR 0x04 ; red on black
-%define PRINT_COLOR 0x07 ; grey on black
+%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 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
+mov ch, 0x3f ; cursor start and options
+mov ah, 0x01 ; set text-mode cursor shape
+int 0x10 ; video services
-; clear video memory
-mov cx, VGA_LENW
-mov ax, VGA_SEG
-mov es, ax
-xor di, di
-mov ax, (FILL_COLOR << 8) | FILL_CHAR
+; 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
-rep stosw ; fill cx words at es:di with ax
+init_grid:
-; errors ----------------------------------------------------------------------
+mov ah, PRINT_COLOR ; ah = color attribute
+mov di, [currvgapg] ; es:di -> grid start
+mov cx, COLS * ROWS ; for each cell
-hello:
-mov si, str.hello
+.write_cell: ; do {
+ call xs ; [xss] = rand
+ mov al, DEAD ; al = DEAD (likely)
-; 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
+ test word [xss], 0b11
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+ jnz .nz ; if ([xss] % 4 == 0)
+ mov al, ALIVE ; al = ALIVE
-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
+.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 ?
+
+alive_neighbours:
+
+; todo
+
+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 ?
+
+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 1f
+
+test ax, 2 ; if (n < 2)
+jl .dead ; return DEAD
+
+test ax, 3 ; if (n > 3)
+jg .dead ; return DEAD
+
+jmp .alive ; return ALIVE
+
+1: ; }
+
+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, cx
+
+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
+
+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