Sprite port-mirror Attribute 3 Register

Number	$38
Readable	No
Writable	Yes
Short Description	Nextreg port-mirror to write directly into "byte 4" of Sprite Attribute Upload ($xx57 / 87).

Byte 4 is bitmapped:

Bit	Description
7	Enable visibility
6	? see note below.
5-0	Pattern index ("Name")

Bit 6 is in normal port writing used to indicate there will be "byte 5" following, while with NextReg mirror the "byte 5" is directly accessible at Sprite port-mirror Attribute 4 Register ($39). At the time of adding this documentation, it is not clear what bit 6 does here, but value "0" may work as "reset whole byte 5 to zero", so if you intend to use extended "byte 5" features, write "1" or test with actual HW. And other way too, if you don't plan to use the extra features, probably write zero to Sprite port-mirror Attribute 4 Register ($39) explicitly, even if you use "0" for bit 6 here.