From b79bf2fbda19b76e81bdc79dd4e8910ab3cc235c Mon Sep 17 00:00:00 2001
From: Vince Weaver <vincent.weaver@maine.edu>
Date: Thu, 21 Dec 2017 13:57:29 -0500
Subject: [PATCH] mode7: update checkerboard demo to have scrolling sky

---
 asm_routines/bg_scroll.s | 79 ++++++++++++++++++++++++++++++++++++++++
 mode7/Makefile           |  4 +-
 mode7/mode7.s            | 31 +++++++++++++---
 3 files changed, 108 insertions(+), 6 deletions(-)
 create mode 100644 asm_routines/bg_scroll.s

diff --git a/asm_routines/bg_scroll.s b/asm_routines/bg_scroll.s
new file mode 100644
index 00000000..64444f24
--- /dev/null
+++ b/asm_routines/bg_scroll.s
@@ -0,0 +1,79 @@
+	;==================
+	; scroll background
+	;==================
+	; background already loaded
+	; ANGLE 0-15 sets angle
+	; CV is Y position to display at
+	; 182/220... 220/16 = 13.75
+	; 0 1   2  3  4  5  6  7  8   9  10  11  12  13  14  15  16
+	; 0 11 22 34 45 56 68 79 91 102 113 125 136 147 159 170 182
+
+scroll_offsets:
+	.byte 0,11,22,34,45,56,68,79,91,102,113,125,136,147,159,170
+
+scroll_background:
+	ldy	ANGLE			; use angle
+	lda	scroll_offsets,Y	; lookup in table
+	sta	OFFSET			; calculate offset
+
+	ldx	#0
+
+	ldy	CV			; lookup Y co-ord
+	lsr				; and set up self-modify code
+
+	lda	gr_offsets,Y		; get position
+	sta	bgsm1+1
+	lda	gr_offsets+2,Y		; get position
+	sta	bgsm2+1
+	lda	gr_offsets+4,Y		; get position
+	sta	bgsm3+1
+	lda	gr_offsets+6,Y		; get position
+	sta	bgsm4+1
+
+
+	iny
+
+	clc
+
+	lda	gr_offsets,Y		; get position
+	adc	DRAW_PAGE
+	sta	bgsm1+2
+	lda	gr_offsets+2,Y		; get position
+	adc	DRAW_PAGE
+	sta	bgsm2+2
+	lda	gr_offsets+4,Y		; get position
+	adc	DRAW_PAGE
+	sta	bgsm3+2
+	lda	gr_offsets+6,Y		; get position
+	adc	DRAW_PAGE
+	sta	bgsm4+2
+
+	ldy	OFFSET
+
+
+bgdraw_loop:
+
+	lda	scroll_row1,Y
+bgsm1:
+	sta	$400,X
+
+	lda	scroll_row2,Y
+bgsm2:
+	sta	$480,X
+
+	lda	scroll_row3,Y
+bgsm3:
+	sta	$500,X
+
+	lda	scroll_row4,Y
+bgsm4:
+	sta	$580,X
+
+	iny
+	inx
+	cpx	#40
+	bne	bgdraw_loop
+
+	rts
+
+
diff --git a/mode7/Makefile b/mode7/Makefile
index 82f473e3..360d6ce0 100644
--- a/mode7/Makefile
+++ b/mode7/Makefile
@@ -45,7 +45,9 @@ mode7_checkerboard.o:	mode7.s checkerboard_lookup.s \
 	../asm_routines/gr_setpage.s \
 	../asm_routines/keypress.s \
 	../asm_routines/gr_putsprite.s \
-	../asm_routines/text_print.s
+	../asm_routines/text_print.s \
+	../asm_routines/gr_scroll.s \
+	../asm_routines/bg_scroll.s
 	ca65 -o mode7_checkerboard.o mode7.s -D CHECKERBOARD_MAP=1 -l mode7.lst
 
 MODE7_RAINBOW:	mode7_rainbow.o
diff --git a/mode7/mode7.s b/mode7/mode7.s
index 0b00e9cc..bfed0d14 100644
--- a/mode7/mode7.s
+++ b/mode7/mode7.s
@@ -32,6 +32,14 @@ CONST_LOWRES_HALF_F	EQU	$00
 	; Initialize the 2kB of multiply lookup tables
 	jsr	init_multiply_tables
 
+.if .def(CHECKERBOARD_MAP)
+	lda	#>sky_background
+	sta	INH
+	lda	#<sky_background
+	sta	INL
+	jsr	decompress_scroll
+.endif
+
 	;===============
 	; Init Variables
 	;===============
@@ -603,11 +611,6 @@ update_z_factor:
 
 draw_background_mode7:
 
-
-.if .def(CHECKERBOARD_MAP)
-
-.else
-
 	; Only draw sky if necessary
 	; (at start, or if we have switched to text, we never overwrite it)
 
@@ -615,6 +618,14 @@ draw_background_mode7:
 	beq	no_draw_sky						;^2nt/3
 								;==============
 								;	  6
+
+.if .def(CHECKERBOARD_MAP)
+	lda	#0
+	sta	CV
+
+	jsr	scroll_background
+
+.else
 	; Draw Sky
 	; not performance critical as this happens rarely
 
@@ -1110,6 +1121,12 @@ exit:
 .include "../asm_routines/gr_putsprite.s"
 .include "../asm_routines/text_print.s"
 
+.if .def(CHECKERBOARD_MAP)
+.include "../asm_routines/gr_scroll.s"
+.include "../asm_routines/bg_scroll.s"
+.endif
+
+
 .include "sprites.inc"
 
 ;===============================================
@@ -1120,6 +1137,10 @@ exit:
 .include "island_map.inc"
 .endif
 
+.if .def(CHECKERBOARD_MAP)
+.include "starry_sky.scroll"
+.endif
+
 .include "../asm_routines/multiply_fast.s"