1 files changed, 167 insertions, 0 deletions
diff --git a/src/home/scroll.asm b/src/home/scroll.asm
new file mode 100644
index 0000000..94896cf
--- /dev/null
+++ b/src/home/scroll.asm
@@ -0,0 +1,167 @@
+; something window scroll
+Func_3e44:
+	push af
+	push hl
+	push bc
+	push de
+	ld hl, wd657
+	bit 0, [hl]
+	jr nz, .done
+	set 0, [hl]
+	ld b, $00
+	ld hl, wd658
+	ld c, [hl]
+	inc [hl]
+	ld hl, wd64b
+	add hl, bc
+	ld a, [hl]
+	ldh [rWX], a
+	ld hl, rLCDC
+	cp $a7
+	jr c, .disable_sprites
+	set 1, [hl] ; enable sprites
+	jr .asm_3e6c
+.disable_sprites
+	res 1, [hl] ; disable sprites
+.asm_3e6c
+	ld hl, wd651
+	add hl, bc
+	ld a, [hl]
+	cp $8f
+	jr c, .asm_3e9a
+	ld a, [wd665]
+	or a
+	jr z, .asm_3e93
+	ld hl, wd659
+	ld de, wd64b
+	ld bc, $6
+	call CopyDataHLtoDE
+	ld hl, wd65f
+	ld de, wd651
+	ld bc, $6
+	call CopyDataHLtoDE
+.asm_3e93
+	xor a
+	ld [wd665], a
+	ld [wd658], a
+.asm_3e9a
+	ldh [rLYC], a
+	ld hl, wd657
+	res 0, [hl]
+.done
+	pop de
+	pop bc
+	pop hl
+	pop af
+	ret
+
+; apply background scroll for lines 0 to 96 using the values at BGScrollData
+; skip if wApplyBGScroll is non-0
+ApplyBackgroundScroll:
+	push af
+	push hl
+	call DisableInt_LYCoincidence
+	ld hl, rSTAT
+	res STAT_LYCFLAG, [hl] ; reset coincidence flag
+	ei
+	ld hl, wApplyBGScroll
+	ld a, [hl]
+	or a
+	jr nz, .done
+	inc [hl]
+	push bc
+	push de
+	xor a
+	ld [wNextScrollLY], a
+.ly_loop
+	ld a, [wNextScrollLY]
+	ld b, a
+.wait_ly
+	ldh a, [rLY]
+	cp $60
+	jr nc, .ly_over_0x60
+	cp b ; already hit LY=b?
+	jr c, .wait_ly
+	call GetNextBackgroundScroll
+	ld hl, rSTAT
+.wait_hblank_or_vblank
+	bit STAT_BUSY, [hl]
+	jr nz, .wait_hblank_or_vblank
+	ldh [rSCX], a
+	ldh a, [rLY]
+	inc a
+	ld [wNextScrollLY], a
+	jr .ly_loop
+.ly_over_0x60
+	xor a
+	ldh [rSCX], a
+	ld a, $00
+	ldh [rLYC], a
+	call GetNextBackgroundScroll
+	ldh [hSCX], a
+	pop de
+	pop bc
+	xor a
+	ld [wApplyBGScroll], a
+	call EnableInt_LYCoincidence
+.done
+	pop hl
+	pop af
+	ret
+
+BGScrollData:
+	db  0,  0,  0,  1,  1,  1,  2,  2,  2,  3,  3,  3,  3,  3,  3,  3
+	db  4,  3,  3,  3,  3,  3,  3,  3,  2,  2,  2,  1,  1,  1,  0,  0
+	db  0, -1, -1, -1, -2, -2, -2, -3, -3, -3, -4, -4, -4, -4, -4, -4
+	db -5, -4, -4, -4, -4, -4, -4, -3, -3, -3, -2, -2, -2, -1, -1, -1
+; 3f38
+
+; x = BGScrollData[(wVBlankCounter + a) & $3f]
+; return, in register a, x rotated right [wBGScrollMod]-1 times (max 3 times)
+GetNextBackgroundScroll:
+	ld hl, wVBlankCounter
+	add [hl]
+	and $3f
+	ld c, a
+	ld b, $00
+	ld hl, BGScrollData
+	add hl, bc
+	ld a, [wBGScrollMod]
+	ld c, a
+	ld a, [hl]
+	dec c
+	jr z, .done
+	dec c
+	jr z, .halve
+	dec c
+	jr z, .quarter
+; effectively zero
+	sra a
+.quarter
+	sra a
+.halve
+	sra a
+.done
+	ret
+
+; enable lcdc interrupt on LYC=LC coincidence
+EnableInt_LYCoincidence:
+	push hl
+	ld hl, rSTAT
+	set STAT_LYC, [hl]
+	xor a
+	ld hl, rIE
+	set INT_LCD_STAT, [hl]
+	pop hl
+	ret
+
+; disable lcdc interrupt and the LYC=LC coincidence trigger
+DisableInt_LYCoincidence:
+	push hl
+	ld hl, rSTAT
+	res STAT_LYC, [hl]
+	xor a
+	ld hl, rIE
+	res INT_LCD_STAT, [hl]
+	pop hl
+	ret