Changes
* Reading from the Kanji ROM ports without a Kanji ROM returns databus values
* Reading from the write only ports (or unused ports) returns databus values.
* Palette red data has a mask of &9F (bits red and color key bit), blue and green have a mask of 0x01f.
* If a command transfers data, and it requests data to be written to the command data port, then you can't use a read of this port to clear the data request. It must be a write. Similarly, if a command requests data to be read then you can use a write to 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.
