| 7:0 | Mask indicating which bits of an attribute byte are used to represent INK.
Other bits represent PAPER. (soft reset = 0x07)
The mask can only indicate a solid sequence of bits on the right side of an attribute
byte (1, 3, 7, 15, 31, 63, 127 or 255).
INKs are mapped to base index 0 in the palette and PAPERs and border are
mapped to base index 128 in the palette.
The 255 value enables the full ink colour mode making all the palette entries INK.
In this case PAPER and border are both taken from the fallback colour in nextreg 0x4A.
If the mask is not one of those listed above, the INK is taken as the logical AND of
the mask with the attribute byte and the PAPER and border colour are again both taken
from the fallback colour in nextreg 0x4A. |