initialize grid

[lifeboot.git] / lifeboot.asm

diff --git a/lifeboot.asm b/lifeboot.asm
index a3b4658fbbf6f6c1839718c149188cc457a8587a..b0575aedc345a986800531d5002f8330be49286e 100644 (file)
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -1,16 +1,19 @@
 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 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 -----------------------------------------------------------------
 
@@ -32,14 +35,71 @@ mov ah, 0x01
 int 0x10
 
 ; clear video memory
-mov cx, VGA_LENW
-mov ax, VGA_SEG
+mov cx, COLS * ROWS
+mov ax, VGA >> 4
 mov es, ax
 xor di, di
-mov ax, (FILL_COLOR << 8) | FILL_CHAR
+mov ax, (PRINT_COLOR << 8) | DEAD
 
 rep stosw                   ; fill cx words at es:di with ax
 
+; setup -----------------------------------------------------------------------
+
+setup:
+
+; initialize xss
+mov ah, 0x00                ; get
+int 0x1a                    ;  system time
+mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
+
+; initialize grid
+mov di, GRID
+mov cx, COLS * ROWS
+
+.initgrid:                  ; do {
+    call xs                 ;     ax = rand
+    mov dl, DEAD            ;     dl = DEAD (likely)
+
+    test ax, 0b11
+
+    jnz  .initgrid_nz       ;     if (ax % 4 == 0)
+    mov dl, ALIVE           ;         dl = ALIVE
+
+.initgrid_nz:
+    mov [di + cx], dl       ;     grid[cx] = dl
+
+    loop .initgrid          ; } while (--cx)
+
+jmp halt
+
+; functions -------------------------------------------------------------------
+
+; xs() - xorshift pseudorandom number generator -------------------------------
+
+; output:
+; ax        = updated state ([XSS])
+
+; clobbers ax, dx
+
+xs:
+
+mov ax, [XSS]
+mov dx, ax
+
+shl dx, 1                   ; dx = xss << 1
+xor ax, dx                  ; ax = xss ^ (xss << 1)
+mov dx, ax
+
+shr dx, 3                   ; dx = xss' >> 3
+xor ax, dx                  ; ax = xss' ^ (xss' >> 3)
+mov dx, ax
+
+shl dx, 10                  ; dx = xss'' << 10
+xor ax, dx                  ; ax = xss'' ^ (xss'' << 10)
+mov [XSS], ax
+
+ret
+
 ; errors ----------------------------------------------------------------------
 
 hello: