bits 16
org 0x7c00
-%define ORG 0x7c00
-%define VGA 0xb8000
+; =============================================================================
+
+; 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
+
+; constants -------------------------------------------------------------------
%define COLS 80
%define ROWS 25
+%define VGAPGSZ (COLS * ROWS * 2)
+
+; memory layout ---------------------------------------------------------------
-%define GRID ORG + 512
-%define NEXT_GRID GRID_CUR + COLS * ROWS
+%define ORG 0x7c00
+%define VGA 0xb8000
+%define DAT ORG + 512
-%define DEAD ' '
-%define ALIVE '#'
+; static variables ------------------------------------------------------------
-%define PRINT_COLOR 0x07 ; grey on black
+%define xss DAT ; xs() state (word)
+%define currvgapg xss + 2 ; current vga page near pointer (word)
+
+; =============================================================================
; entry point -----------------------------------------------------------------
; initialize segment registers
-xor ax, ax
+mov ax, VGA >> 4
mov ds, ax
mov es, ax
+
+xor ax, 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, COLS * ROWS
-mov ax, VGA >> 4
-mov es, ax
-xor di, di
-mov ax, (PRINT_COLOR << 8) | DEAD
+; 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
+
+; setup -----------------------------------------------------------------------
+
+setup:
+
+call init_grid
+call flip_vga_page
+call delay
+
+jmp setup
+
+; =============================================================================
+
+; 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 for display to enter the next VBlank cycle -------------------
-rep stosw ; fill cx words at es:di with ax
+; clobbers ax, dx
-; errors ----------------------------------------------------------------------
+vsync:
-hello:
-mov si, str.hello
+mov dx, 0x3da ; input status #1 register
-; 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
+.wait_on:
+ in al, dx
+ test al, 0x08 ; vertical retrace bit
+ jnz .wait_on
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+.wait_off:
+ in al, dx
+ test al, 0x08
+ jz .wait_off
-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
+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
+
+xor word [currvgapg], VGAPGSZ ; flip currvgapg
+setnz al ; al = !(currvgapg == 0)
+
+mov ah, 0x05 ; select active display page
+int 0x10 ; video services
+
+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