GFXMODE
         16bit register selecting the screen size and the pixelformat for the Chunky plane.
         

       +-------+----------+-------------------------------------------+
         | BIT#  | FUNCTION | DESCRIPTION                             |
         +-------+----------+-----------------------------------------+
         | 15.08 | Resolution  | $00                                  |
         |       |               $01 = 320x200                        |
         |       |               $02 = 320x240                        |
         |       |               $03 = 320x256                        |
         |       |               $04 = 640x400                        |
         |       |               $05 = 640x480                        |
         |       |               $06 = 640x512                        |
         |       |               $07 = 960x540                        |
         |       |               $08 = 480x270                        |
         |       |               $09 = 304x224                        |
         |       |               $0A = 1280x720                       |
         |       |               $0B = 640x360                        |
         |       |               $0C = 800x600                        |
         |       |               $0D = 1024x768                       |
         |       |               $0E = 720x576                        |
         |       |               $0F = 848x480                        |
         +-------+----------+-----------------------------------------+ 
         | 07.00 | Pixelformat | $00                                  |
         |       |               $01 = 8bit (indexed)                 |
         |       |               $02 = 16bit R5G6B5                   |
         |       |               $03 = 15bit 1R5G5B5                  |
         |       |               $04 = 24bit R8G8B8                   |
         |       |               $05 = 32bit A8R8G8B8                 |
         |       |               $06 = YUV                            |
         |       |               $07 =                                |
         |       |               $08 = PLANAR 1BIT                    |
         |       |               $09 = PLANAR 2BIT                    |
         |       |               $0A = PLANAR 4BIT                    |
         +-------+----------+-----------------------------------------+