When UHRES is enabled, this pointer comes out on the 2nd 'free' cycle
         after the start of each horizontal line. It`s modulo is added every
         time it comes out. 'free' means priority above the copper and below
         the fixed stuff (audio,sprites....).
         BPLHDAT comes out as an identifier on the RGA lines when the pointer
         address is valid so that external detectors can use this to do the
         special cycle for the VRAMs, The SPRHDAT gets the first and third
         free cycles.
Interesting links: