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)
+
+; 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
-lds ax, [farptr.zero]
-les ax, [farptr.zero]
-lss ax, [farptr.zero]
+mov ax, VGA >> 4
+mov ds, ax
+mov es, ax
+
+xor ax, ax
+mov ss, ax
; set stack pointers
mov bp, ORG
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
; 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 currvgapg
+mov word [currvgapg], VGAPGSZ
; setup -----------------------------------------------------------------------
setup:
call init_grid
-call print_grid
+call flip_vga_page
call delay
jmp setup
+; =============================================================================
+
; functions -------------------------------------------------------------------
; delay() - suspend program execution temporarily -----------------------------
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
+; [xss] = updated state
; clobbers bx, dx
xs:
-mov bx, [XSS]
+mov bx, [xss]
mov dx, bx
shl dx, 1 ; dx = xss << 1
shl dx, 10 ; dx = xss'' << 10
xor bx, dx ; bx = xss'' ^ (xss'' << 10)
-mov [XSS], bx
+mov [xss], bx
ret
-; init_grid() - initialize GRID cells with random values ----------------------
-
-; (*) expects es = 0.
+; init_grid() - initialize grid cells randomly --------------------------------
-; clobbers al, bx, cx, dx, di
+; clobbers ax, bx, cx, dx, di
init_grid:
-mov di, GRID ; es:di -> 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
+ call xs ; [xss] = rand
mov al, DEAD ; al = DEAD (likely)
- test word [XSS], 0b11
+ test word [xss], 0b11
- jnz .nz ; if ([XSS] % 4 == 0)
+ jnz .nz ; if ([xss] % 4 == 0)
mov al, ALIVE ; al = ALIVE
.nz:
- stosb ; [es:(di++)] = al
+ stosw ; [es:di] = ax ; di += 2
loop .write_cell ; } while (--cx)
ret
-; print_grid() - print GRID cells to screen -----------------------------------
+; flip_vga_page() - flip active display page ----------------------------------
-; (*) this function expects ds = 0 when called and sets es = 0 before
-; returning. this is simpler because in the rest of the program we want the
-; segment registers to be zeroed anyway.
+; clobbers ax, dx
-; clobbers ax, cx, di, si
+flip_vga_page:
-print_grid:
+call vsync_wait
-les di, [farptr.vga_start] ; es:di -> VGA
-mov si, GRID ; ds:si -> GRID
+xor word [currvgapg], VGAPGSZ ; flip currvgapg
+setnz al ; al = !(currvgapg == 0)
-mov ah, PRINT_COLOR ; ah = color attribute
-
-mov cx, COLS * ROWS ; for each cell
+mov ah, 0x05 ; select active display page
+int 0x10 ; video services
-.print_cell: ; do {
-
- lodsb ; al = [ds:(si++)]
- stosw ; [es:di] = ax ; di += 2
-
- loop .print_cell ; } while (--cx)
-
-mov es, cx ; es = 0
ret
; halt() - stop program execution ---------------------------------------------
hlt
jmp halt
-; data ------------------------------------------------------------------------
-
-farptr: ; offset, segment
-.zero:
- dw 0 ; take advantage of next dw 0
-
-.vga_start:
- dw 0, VGA >> 4
+; =============================================================================
times 510 - ($ - $$) db 0 ; fill remaining bytes with zeroes
dw 0xaa55 ; mbr magic byte
projects / lifeboot.git / blobdiff