1 files changed, 93 insertions, 0 deletions
diff --git a/src/home/switch_rom.asm b/src/home/switch_rom.asm
new file mode 100644
index 0000000..ab0736f
--- /dev/null
+++ b/src/home/switch_rom.asm
@@ -0,0 +1,93 @@
+; switch to rombank (a + top2 of h shifted down),
+; set top2 of h to 01 (switchable ROM bank area),
+; return old rombank id on top-of-stack
+BankpushROM:
+	push hl
+	push bc
+	push af
+	push de
+	ld e, l
+	ld d, h
+	ld hl, sp+$9
+	ld b, [hl]
+	dec hl
+	ld c, [hl]
+	dec hl
+	ld [hl], b
+	dec hl
+	ld [hl], c
+	ld hl, sp+$9
+	ldh a, [hBankROM]
+	ld [hld], a
+	ld [hl], $0
+	ld a, d
+	rlca
+	rlca
+	and $3
+	ld b, a
+	res 7, d
+	set 6, d ; $4000 ≤ de ≤ $7fff
+	ld l, e
+	ld h, d
+	pop de
+	pop af
+	add b
+	call BankswitchROM
+	pop bc
+	ret
+
+; switch to rombank a,
+; return old rombank id on top-of-stack
+BankpushROM2:
+	push hl
+	push bc
+	push af
+	push de
+	ld e, l
+	ld d, h
+	ld hl, sp+$9
+	ld b, [hl]
+	dec hl
+	ld c, [hl]
+	dec hl
+	ld [hl], b
+	dec hl
+	ld [hl], c
+	ld hl, sp+$9
+	ldh a, [hBankROM]
+	ld [hld], a
+	ld [hl], $0
+	ld l, e
+	ld h, d
+	pop de
+	pop af
+	call BankswitchROM
+	pop bc
+	ret
+
+; restore rombank from top-of-stack
+BankpopROM:
+	push hl
+	push de
+	ld hl, sp+$7
+	ld a, [hld]
+	call BankswitchROM
+	dec hl
+	ld d, [hl]
+	dec hl
+	ld e, [hl]
+	inc hl
+	inc hl
+	ld [hl], e
+	inc hl
+	ld [hl], d
+	pop de
+	pop hl
+	pop af
+	ret
+
+; switch ROM bank to a
+BankswitchROM:
+	ldh [hBankROM], a
+	ld [MBC3RomBank], a
+	ret