diff --git a/BESS.md b/BESS.md
index 59ca3db..03cb54c 100644
--- a/BESS.md
+++ b/BESS.md
@@ -74,6 +74,7 @@ The values of memory-mapped registers should be written 'as-is' to memory as if
 * Unused registers have Don't-Care values which should be ignored
 * Unused register bits have Don't-Care values which should be ignored
 * If the model is CGB or newer, the value of KEY0 (FF4C) must be valid as it determines DMG mode
+    * Bit 2 determines DMG mode. A value of 0x04 usually denotes DMG mode, while a value of `0x80` usually denotes CGB mode.
 * Sprite priority is derived from KEY0 (FF4C) instead of OPRI (FF6C) because OPRI can be modified after booting, but only the value of OPRI during boot ROM execution takes effect
 * If a register doesn't exist on the emulated model (For example, KEY0 (FF4C) on a DMG), its value should be ignored.
 * BANK (FF50) should be 0 if the boot ROM is still mapped, and 1 otherwise, and must be valid.