V9990

• 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 (SCREEN MODE): Mask is &FF.
  - 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