second: add falling bars during interference

demos/second/part04_chess_shapes/Makefile

@@ -19,6 +19,7 @@ chess.o:	chess.s \
 	../hgr_copy_fast.s \
 	interference.s \
 	circles.s \
+	falling_bars.s \
 	sierzoom.s \
 	../irq_wait.s \
 	../zp.inc ../hardware.inc ../qload.inc \

demos/second/part04_chess_shapes/chess.s

@@ -320,9 +320,45 @@ main_tunnel_done:
 	sta	interference_end_smc+1
 	jsr	interference
 
+	;======================
 	; TODO: falling bars
+	;======================
+	lda	#0
+	sta	DRAW_PAGE
+	bit	PAGE1
 	jsr	clear_all
-	lda	#50
+
+	; left
+
+	lda	#$FF
+	sta	BAR_X1
+	lda	#13
+	sta	BAR_X2
+	jsr	falling_bars
+
+	lda	#3
+	jsr	wait_ticks
+
+	; middle
+
+	lda	#13
+	sta	BAR_X1
+	lda	#27
+	sta	BAR_X2
+	jsr	falling_bars
+
+	lda	#3
+	jsr	wait_ticks
+
+	; right
+
+	lda	#27
+	sta	BAR_X1
+	lda	#39
+	sta	BAR_X2
+	jsr	falling_bars
+
+	lda	#5
 	jsr	wait_ticks
 
 
@@ -360,6 +396,7 @@ main_interference_done:
 	.include	"circles.s"
 	.include	"sierzoom.s"
 	.include	"../hgr_page_flip.s"
+	.include	"falling_bars.s"
 
 	; wait A * 1/50s
 wait_irq:

demos/second/part04_chess_shapes/falling_bars.s

@@ -0,0 +1,30 @@
+
+	;==============================
+	; falling bars
+	;==============================
+	; set X1 and X2 before calling
+falling_bars:
+
+	ldx	#0
+falling_bar_outer:
+	lda	gr_offsets_l,X
+	sta	GBASL
+	lda	gr_offsets_h,X
+	sta	GBASH
+
+	ldy	BAR_X2
+	lda	#0
+falling_bar_inner:
+	sta	(GBASL),Y
+	dey
+	cpy	BAR_X1
+	bne	falling_bar_inner
+
+	lda	#50
+	jsr	wait
+
+	inx
+	cpx	#24
+	bne	falling_bar_outer
+
+	rts

demos/second/zp.inc

@@ -198,7 +198,8 @@ T_L     =       $F6
 T_H     =       $F7
 SAVED   =       $F8
 
-
+BAR_X1 =	$F0
+BAR_X2	=	$F1
 
 ;==============================================
 ; $FC-$FF we use for in/out pointers