
SCREEN ADDITION/SUBTRACTION

[...]

Function :
This function indicates the result after the addition or the
subtraction of RGB data on the main screen and sub screen.
This function can also select BG screen or OBJ data on the main
screen to be added to or subtracted from the sub screen similar
to the fig. below

However, when there is no screen data on the sub screen (screen is clear), 
the color constant explained on page 15 will be added or subtracted.

When the result added or subtracted on each RGB is over 31 the value becomes
31, also, when the value added or subtracted on each RGB is under 0, the 
value becomes 0.
Please don't use this function on BG mode 5 or 6
                                                ______________________
                                               |                     |
   MAIN SCREEN  ------------------------------>|ADDITION/SUBTRACTION |
      DATA              |                      |      Select         |
                        |                      |_____________________|
                        |
                        |
                        |                       _________________
                        |--------------------->|                |
                                               |Adder-Subtracter|
   SUB SCREEN -------------------------------->|                | -----> DISPLAY
      DATA                                     |________________|



SETTING EXAMPLE :

 INITIAL SETTINGS :

  * clear each register
  * enable BG to display
  * enable OBJ to display
  * Set D1 of register <2130H>
     "CC ADD ENABLE" Settings
  * Set Register <2130H>
    ADD or SUB Enable     -|
    1/2 Enable             | Settings
    ADD/SUB               -|
  * Set register <212DH>
    "Through Sub" settings
                 |
   ------------->|
   |             |
   |          DISPLAY
   |             |
   |_____________|


Note : When main screen data is the obj, it'll be added to or subtracted
       from the sub screen data only for the OBJ of the palette code (4 to 7)
       It is convenient for generating the dusky shadow

Note : When "1/2 Enable" of register <2130> is enabled, the addition/subtrct.
       result of each RGB becomes 1/2. 
       