added xmacro.h

machines/atari2600/xmacro.h

@@ -0,0 +1,25 @@
+
+;-------------------------------------------------------
+; Usage: TIMER_SETUP lines
+; where lines is the number of scanlines to skip (> 2).
+; The timer will be set so that it expires before this number
+; of scanlines. A WSYNC will be done first.
+
+    MAC TIMER_SETUP
+.lines  SET {1}
+        lda #(((.lines-1)*76-14)/64)
+        sta WSYNC
+        sta TIM64T
+    ENDM
+
+;-------------------------------------------------------
+; Use with TIMER_SETUP to wait for timer to complete.
+; You may want to do a WSYNC afterwards, since the timer
+; is not accurate to the beginning/end of a scanline.
+
+    MAC TIMER_WAIT
+.waittimer
+        lda INTIM
+        bne .waittimer
+    ENDM
+