Documented SwScrl_ARZ

Some neat beta stuff in here
sonicretro · Jun 10, 2016 · d2af3c2 · d2af3c2
1 parent 9b133e9
commit d2af3c2
Showing 1 changed file with 58 additions and 31 deletions.
diff --git a/s2.asm b/s2.asm
@@ -16160,25 +16160,32 @@ SwScrl_ARZ:
 +
 	moveq	#6,d6
 	bsr.w	SetVertiScrollFlagsBG
+
 	move.w	(Camera_BG_Y_pos).w,(Vscroll_Factor_BG).w
+
 	moveq	#0,d2
 	tst.b	(Screen_Shaking_Flag).w
-	beq.s	+
+	beq.s	.screenNotShaking
 
 	move.w	(Timer_frames).w,d0
 	andi.w	#$3F,d0
 	lea_	SwScrl_RippleData,a1
 	lea	(a1,d0.w),a1
 	moveq	#0,d0
+	; Shake camera Y-pos (note that BG scrolling is not affected by this, causing it to distort)
 	move.b	(a1)+,d0
 	add.w	d0,(Vscroll_Factor_FG).w
 	add.w	d0,(Vscroll_Factor_BG).w
 	add.w	d0,(Camera_Y_pos_copy).w
+	; Shake camera X-pos
 	move.b	(a1)+,d2
 	add.w	d2,(Camera_X_pos_copy).w
-+
-	lea	(TempArray_LayerDef).w,a2
-	lea	6(a2),a3
+
+.screenNotShaking:
+	lea	(TempArray_LayerDef).w,a2	; Starts at BG scroll row 1
+	lea	6(a2),a3			; Starts at BG scroll row 4
+
+	; Set up the speed of each row (there are 16 rows in total)
 	move.w	(Camera_X_pos).w,d0
 	ext.l	d0
 	asl.l	#4,d0
@@ -16187,61 +16194,81 @@ SwScrl_ARZ:
 	asl.l	#4,d0
 	asl.l	#8,d0
 	move.l	d0,d1
+
+	; Set row 4's speed
 	swap	d1
-	move.w	d1,(a3)+
+	move.w	d1,(a3)+	; Top row of background moves 10 ($A) times slower than foreground
 	swap	d1
 	add.l	d1,d1
 	add.l	d0,d1
+	; Set rows 5-10's speed
     rept 6
 	swap	d1
-	move.w	d1,(a3)+
+	move.w	d1,(a3)+	; Next row moves 3 times faster than top row, then next row is 4 times faster, then 5, etc.
 	swap	d1
 	add.l	d0,d1
     endm
+	; Set row 11's speed
 	swap	d1
 	move.w	d1,(a3)+
-	move.w	d1,(a2)
-	move.w	d1,4(a2)
+
+	; These instructions reveal that ARZ had slightly different scrolling,
+	; at one point:
+	; Above the background's mountains is a row of leaves, which is actually
+	; composed of three separately-scrolling rows. According to this code,
+	; the first and third rows were meant to scroll at a different speed to the
+	; second. Possibly due to how bad it looks, the speed values are overwritten
+	; a few instructions later, so all three move at the same speed.
+	; This code seems to pre-date the Simon Wai build, which uses the final's
+	; scrolling.
+	move.w	d1,(a2)		; Set row 1's speed
+	move.w	d1,4(a2)	; Set row 3's speed
+
 	move.w	(Camera_BG_X_pos).w,d0
-	move.w	d0,2(a2)
-	move.w	d0,$16(a2)
-	_move.w	d0,0(a2)
-	move.w	d0,4(a2)
-	move.w	d0,$18(a2)
-	move.w	d0,$1A(a2)
-	move.w	d0,$1C(a2)
-	move.w	d0,$1E(a2)
-	lea	(byte_D5CE).l,a3
+	move.w	d0,2(a2)	; Set row 2's speed
+	move.w	d0,$16(a2)	; Set row 12's speed
+	_move.w	d0,0(a2)	; Overwrite row 1's speed (now same as row 2's)
+	move.w	d0,4(a2)	; Overwrite row 3's speed (now same as row 2's)
+	move.w	d0,$18(a2)	; Set row 13's speed
+	move.w	d0,$1A(a2)	; Set row 14's speed
+	move.w	d0,$1C(a2)	; Set row 15's speed
+	move.w	d0,$1E(a2)	; Set row 16's speed
+
+	lea	(SwScrl_ARZ_RowHeights).l,a3
 	lea	(TempArray_LayerDef).w,a2
 	lea	(Horiz_Scroll_Buf).w,a1
 	move.w	(Camera_BG_Y_pos).w,d1
 	moveq	#0,d0
 
--	move.b	(a3)+,d0
-	addq.w	#2,a2
+	; Find which row of background is visible at the top of the screen
+.findTopRowLoop:
+	move.b	(a3)+,d0	; Get row height
+	addq.w	#2,a2		; Next row speed (note: is off by 2. This is fixed below)
 	sub.w	d0,d1
-	bcc.s	-
+	bcc.s	.findTopRowLoop		; If current row is above the screen, loop and do next row
 
-	neg.w	d1
-	subq.w	#2,a2
-	move.w	#bytesToLcnt($380),d2
+	neg.w	d1	; d1 now contains how many pixels of the row is currently on-screen
+	subq.w	#2,a2	; Get correct row speed
+
+	move.w	#bytesToLcnt($380),d2	; Actual size of Horiz_Scroll_Buf
 	move.w	(Camera_X_pos).w,d0
 	neg.w	d0
-	swap	d0
-	move.w	(a2)+,d0
+	swap	d0		; Store FG X-pos in upper 16-bits...
+	move.w	(a2)+,d0	; ...and BG X-pos in lower 16 bits, as Horiz_Scroll_Buf expects it
 	neg.w	d0
 
--	move.l	d0,(a1)+
-	subq.w	#1,d1
+-	move.l	d0,(a1)+	; Write 1 FG Horizontal Scroll value, and 1 BG Horizontal Scroll value
+	subq.w	#1,d1		; Loop until row at top of screen is done
 	bne.s	+
-	move.b	(a3)+,d1
-	move.w	(a2)+,d0
+	move.b	(a3)+,d1	; Once that row is done, go to next row...
+	move.w	(a2)+,d0	; ...and use next speed
 	neg.w	d0
-+	dbf	d2,-
++	dbf	d2,-		; Loop until Horiz_Scroll_Buf is full
 
 	rts
 ; ===========================================================================
-byte_D5CE:
+; byte_D5CE:
+SwScrl_ARZ_RowHeights:
 	dc.b $B0
 	dc.b $70	; 1
 	dc.b $30	; 2