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Topic       : 
Author      : 
Version     : 
Subject     : 68000 machine code
Nodes       : 74
Index Size  : 2808
HCP-Version : 2
Compiled on : Atari
@charset    : atarist
@lang       : 
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@options    : -i -s +x
@width      : 75

           3. 2.  Video Configuration

               The ST possesses three modes  of  video  configuration:
          320  x  200  resolution  with 4 planes, 640 x 200 resolution
          with 2 planes, and 640 x 400 resolution with 1  plane.   The
          modes  are  set through the Shift Mode Register (read/write,
          reset: all zeros).  A sixteen word color lookup  palette  is
          provided  with  nine  bits  of color per entry.  The sixteen
          Color Palette Registers (read/write,  reset:  not  affected)
          contain  three  bits  of red, green, and blue aligned on low
          nibble boundaries.  Eight intensity  levels  of  red,  eight
          intensity  levels  of  green,  and eight intensity levels of
          blue produce a total of 512 possible colors.

               In 320 x 200 4 plane mode all  sixteen  palette  colors
          can  be  indexed,  while  in 640 x 200 2 plane mode only the
          first four palette entries are applicable.   In  640  x  400
          monochrome mode the color palette is bypassed altogether and
          is instead provided with an inverter for inverse video  con-
          trolled  by  bit 0 of palette color 0 (normal video is black
          0, white 1).  Color palette memory is arranged the  same  as
          main  memory.   Palette  color  0  is  also used to assign a
          border color while in a  multi-plane  mode.   In  monochrome
          mode the border color is always black.

               The general flow of the video controller is as follows:
          BitMap  planes are taken a word at a time from video display
          memory and placed in the video shift register where one  bit
          from  each  plane is shifted out and collectively used as an
          index (plane 0 is the least  significant  bit)  to  a  color
          lookup  palette  entry which is supplied to 3 bit digital to
          analog converters to produce RGB output.  The following is a
          block diagram of the video controller:

          ----- Video Controller Block Diagram ---------------

         --------          --------        --------   lsb  --------
        | 3      |---- -->|0 1 2 3 |----->|      0 |----->| 16 x 9 |-
       --------  |    |   |        |      |      1 |----->| Lookup |-|->R
      | 2      |------    |        |      |      2 |----->|        |-
     --------  | |    |   |        |      |      3 |----->|        |-
    | 1      |--------    |        |      |        |--    |        |-|->G
   --------  | |-     |    --------        --------   |   |        |-
  | 0      |----------                                |   |        |-
  |        | |-                                       |   |        |-|->B
  |        | |                                        |   |        |-
  |        |-                                         |    --------
  |        |                                          |    --------
   --------                                            -->|Inverter|->MONO
                                                           --------

  Logical BitMap          Video Display   Video Shift     Color Palette
  Planes                  Memory          Register        and 3 Bit DACs