fix initialize grid

[lifeboot.git] / lifeboot.asm

diff --git a/lifeboot.asm b/lifeboot.asm
index 81185ddd29816e688d1c8ec713df5a4be2cd1fae..8b530f9f378b143a6be628fb119e7692505e95c3 100644 (file)
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -9,7 +9,6 @@ org 0x7c00
 
 %define DEAD ' '            ; char to represent a dead cell
 %define ALIVE '#'           ; char to represent an alive cell
-%define INIT_RATIO 4        ; dead / alive
 %define PRINT_COLOR 0x07    ; grey on black
 
 %define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
@@ -28,6 +27,9 @@ mov ss, ax
 mov sp, ORG
 mov bp, ORG
 
+; clear direction flag
+cld
+
 ; clear screen ----------------------------------------------------------------
 
 ; disable cursor
@@ -53,29 +55,53 @@ mov ah, 0x00                ; get
 int 0x1a                    ;  system time
 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
 
+; initialize grid
+mov cx, COLS * ROWS         ; for each cell
+xor ax, ax
+mov es, ax
+mov di, GRID                ; es:di -> GRID
+
+.initgrid:                  ; do {
+    call xs                 ;     [XSS] = rand
+    mov al, DEAD            ;     al = DEAD (likely)
+
+    test [XSS], 0b11
+
+    jnz  .initgrid_nz       ;     if ([XSS] % 4 == 0)
+    mov al, ALIVE           ;         al = ALIVE
+
+.initgrid_nz:
+    stosb                   ;     [es:(di++)] = al
+
+    loop .initgrid          ; } while (--cx)
+
+jmp halt
+
 ; functions -------------------------------------------------------------------
 
 ; xs() - xorshift pseudorandom number generator -------------------------------
 
 ; output:
-; ax        = updated state ([XSS])
+; [XSS]     = updated state
+
+; clobbers bx, dx
 
 xs:
 
-mov ax, [XSS]
-mov dx, ax
+mov bx, [XSS]
+mov dx, bx
 
 shl dx, 1                   ; dx = xss << 1
-xor ax, dx                  ; ax = xss ^ (xss << 1)
-mov dx, ax
+xor bx, dx                  ; bx = xss ^ (xss << 1)
+mov dx, bx
 
 shr dx, 3                   ; dx = xss' >> 3
-xor ax, dx                  ; ax = xss' ^ (xss' >> 3)
-mov dx, ax
+xor bx, dx                  ; bx = xss' ^ (xss' >> 3)
+mov dx, bx
 
 shl dx, 10                  ; dx = xss'' << 10
-xor ax, dx                  ; ax = xss'' ^ (xss'' << 10)
-mov [XSS], ax
+xor bx, dx                  ; bx = xss'' ^ (xss'' << 10)
+mov [XSS], bx
 
 ret