OSBYTE &53

OSBYTE &53 (83): ADJI KEYV handling

Used by the Retro Hardware ADJI (Advanced Digital Joystick Interface) for hooking into KEYV handling.

Not actually realistically callable via OSBYTE.

The ROM can hook into KEYV and BYTEV, which it does through a little routine stored at a configurable address in memory, bypassing the extended vector mechanism in order to accommodate games that overwrite that region.

To keep the routine small, it funnels both KEYV and OSBYTE calls into the same entry point in the ROM, and &53 - unused on both Master 128 and Electron - indicates that the call is being made through KEYV rather than BYTEV.