bits 16
org 0x7c00
-%define ORG 0x7c00 ; where we are loaded initially
+%define ORG 0x7c00
+%define VGA 0xb8000
-%define VGA_SEG 0xb800 ; video memory starts at 0xb8000
-%define VGA_COL 80
-%define VGA_ROW 25
-%define VGA_LENW VGA_COL * VGA_ROW
+%define COLS 80
+%define ROWS 25
-%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
+
+%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
; entry point -----------------------------------------------------------------
; initialize segment registers
-xor ax, ax
-mov ds, ax
-mov es, ax
-mov ss, ax
+lds ax, [farptr.zero]
+les ax, [farptr.zero]
+lss ax, [farptr.zero]
; 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, 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
+
+; setup -----------------------------------------------------------------------
+
+setup:
+
+call init_grid
+call print_grid
+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
+
+; 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
-rep stosw ; fill cx words at es:di with ax
+ret
-; errors ----------------------------------------------------------------------
+; init_grid() - initialize GRID cells with random values ----------------------
-hello:
-mov si, str.hello
+; (*) expects es = 0.
-; 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
+; clobbers al, bx, cx, dx, di
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+init_grid:
-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 di, GRID ; es:di -> GRID
+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:
+ stosb ; [es:(di++)] = al
+
+ loop .write_cell ; } while (--cx)
+
+ret
+
+; print_grid() - print GRID cells to screen -----------------------------------
+
+; (*) 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, cx, di, si
+
+print_grid:
+
+les di, [farptr.vga_start] ; es:di -> VGA
+mov si, GRID ; ds:si -> GRID
+
+mov ah, PRINT_COLOR ; ah = color attribute
+
+mov cx, COLS * ROWS ; for each cell
+
+.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 ---------------------------------------------
halt:
-cli ; disable interrupts
+
+cli ; disable interrupts
hlt
jmp halt
; data ------------------------------------------------------------------------
-str:
-.hello:
- db "lifeboot", 0
+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