Sprite Attribute 3 (with INC) Register: Difference between revisions

Revision as of 23:19, 9 December 2019

Next Register Number	$78
Readable	No
Writable	Yes
Short Description	Same as NextReg:$38 (write fourth byte of sprite-attributes), plus increments NextReg:$34

Byte 4 is bitmapped:

Bit	Description
7	Enable visibility
6	Enable fifth sprite attribute byte
5-0	Pattern index ("Name")

When the bit 6 is "0", the sprite renderer will ignore value written in fifth attribute byte, for example by NextReg:$39, and it will use zero for all the features of fifth attribute byte (it is safe to initialize only four bytes if you are planning to use 4-byte type of sprites).

If you want the value in fifth attribute byte to apply during rendering, bit 6 here must be set to "1".

After write into sprite-attribute "byte 4" (NextPort:$xx57), the NextReg:$34 is incremented (with all consequences stemming from such action). If the sprite 127 was modified, the result of the increment is officially "undefined behaviour", and code should explicitly set valid 0..127 sprite index before next sprite-attribute manipulation.

@@ Line 11: / Line 11: @@
 | 7 || Enable visibility
 |-
-| 6 || ? see note below.
+| 6 || Enable fifth sprite attribute byte
 |-
 | 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 {{NextRegNo|$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 {{NextRegNo|$39}} explicitly, even if you use "0" for bit 6 here.
+When the bit 6 is "0", the sprite renderer will ignore value written in fifth attribute byte, for example by {{NextRegNo|$39}}, and it will use zero for all the features of fifth attribute byte (it is safe to initialize only four bytes if you are planning to use 4-byte type of sprites).
+If you want the value in fifth attribute byte to apply during rendering, bit 6 here must be set to "1".
 After write into sprite-attribute "byte 4" ({{PortNo|$xx57}}), the {{NextRegNo|$34}} is incremented (with all consequences stemming from such action). If the sprite 127 was modified, the result of the increment is officially "undefined behaviour", and code should explicitly set valid 0..127 sprite index before next sprite-attribute manipulation.