Changes
* Some registers have additional bits which are not documented. The mask describes which bits are read/write and which are unchanged.
Where the mask has a '1' bit, this bit is read/write. Where the mask has a '0' bit this bit remains at 0 and can't be changed.
- Register 7 (SCREEN MODE): Mask is &FF.
- Register 9 (INTERRUPT READ/WRITE): Mask is &87
- Register 15 (BACK DROP COLOR): Mask is &FF
