* 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.
- Register 7: Mask is &7F - Register 9(INTERRUPT READ/WRITE): Mask is &87 - Register 15(BACK DROP COLOR): Mask is &FF - Register 22(SCROLL CONTROL): Mask is &C1 - Register 25(SPRITE PATTERN GENERATOR TABLE BASE ADDRESS): Mask is &CF - Register 26 (LCD CONTROL): Mask is &FF - Register 27(PRIORITY CONTROL): Mask is &FF