Changes
* Reading from the Kanji ROM ports without a Kanji ROM returns databus values
* Palette index 0 has a mask of &9F (bits for r,g,b and color key), other indices have a mask of 0x01f for just r,g,b.
* If a command wants data written to the command port, you can't use a read, this will not clear the data request.
* If you read from a write-only register you will see data-bus value because the V9990 doesn't assert data on the bus.
* 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.
