From c640819179b9ef6f441bd97327634bd70c8842a8 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Fri, 20 Nov 2020 00:35:52 -0800
Subject: [PATCH] GB: Downgrade DMG-only ROMs from CGB mode even without boot
 ROM

---
 CHANGES     | 1 +
 src/gb/gb.c | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/CHANGES b/CHANGES
index 04c6d9c66..aba1aa345 100644
--- a/CHANGES
+++ b/CHANGES
@@ -17,6 +17,7 @@ Emulation fixes:
  - ARM: Fix STR storing PC after address calculation
  - ARM: Fix Addressing mode 1 shifter on rs == pc (fixes mgba.io/i/1926)
  - GB: Partially fix timing for skipped BIOS
+ - GB: Downgrade DMG-only ROMs from CGB mode even without boot ROM
  - GB Audio: Fix serializing sweep time
  - GB MBC: Fix MBC1 mode changing behavior
  - GB MBC: Fix some MBC3 bit masking
diff --git a/src/gb/gb.c b/src/gb/gb.c
index 8e24a21a1..9c13b795b 100644
--- a/src/gb/gb.c
+++ b/src/gb/gb.c
@@ -489,6 +489,11 @@ void GBSkipBIOS(struct GB* gb) {
 	const struct GBCartridge* cart = (const struct GBCartridge*) &gb->memory.rom[0x100];
 	int nextDiv = 0;
 
+	if (gb->model >= GB_MODEL_CGB && !(cart->cgb & 0x80)) {
+		gb->model = GB_MODEL_DMG;
+		GBVideoDisableCGB(&gb->video);
+	}
+
 	switch (gb->model) {
 	case GB_MODEL_AUTODETECT: // Silence warnings
 		gb->model = GB_MODEL_DMG;