Actually update spec
@@ -73,8 +73,9 @@ For example; `'GD '` represents a DMG of an unspecified revision, `'S '` repr
The values of memory-mapped registers should be written 'as-is' to memory as if the actual ROM wrote them, with the following exceptions and note:
* Unused registers have Don't-Care values which should be ignored
* Unused register bits have Don't-Care values which should be ignored
-* The value of KEY0 (FF4C) must be valid as it determines CGB mode
+* The value of KEY0 (FF4C) must be valid as it determines DMG mode when the model is CGB or newer
* 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.
* Implementations should not start a serial transfer when writing the value of SB
* Similarly, no value of NRx4 should trigger a sound pulse on save state load
