The ST`s Screens
================
     Let`s  start  with something we know,  the resolutions &  colors  of  the 
screens:
               Screen   | Resolution| Max Colors | => bits/pixel
             -----------+-----------+------------+---------------
                LOW     | 320 * 200 |     16     |       4
               MEDIUM   | 640 * 200 |      4     |       2
              HI (MONO) | 640 * 400 |      2     |       1


     From this the screen length in bits can be calculated,  and from that the 
length in bytes can be calculated:

                    LOW = (320 * 200 * 4) / 8 = 32000 bytes
                   MED. = (640 * 200 * 2) / 8 = 32000 bytes
                   MONO =   (640 * 400) / 8   = 32000 bytes


Bitplanes
=========
     Starting with MEDIUM res, each pixel(P) needs two bits(B) as follows:

           B2a  B2b
             \  /
          P1, P2, P3 > P16;   P17 > P32;   P33 > P48;   and so on...
         /  \         /   \
       B1a  B1b    B16a   B16b


     These  bits  are not stored consecutively.   Instead they are  stored  in 
     ------------------------------------------
groups of 16, 16 being the ST`s WORD SIZE, as follows:

       +-----Make up P2------+             +-Make up P17--+
       |                     |             |              |
  B1a,B2a,B3a > B16a;   B1b,B2b > B16b;   B17a > B32a;   B17b > B32b;   ...
  ~~~~~~~~~~~~~~~~~~    ~~~~~~~~~~~~~~    ~~~~~~~~~~~    ~~~~~~~~~~~
      Bitplane 1            Plane 2         Plane 1        Plane 2


     In  other  words,  a group of 16`s lower bits are  all  stored  together, 
followed  by that groups higher bits - and these 'groups of bits'  are  called 
BITPLANES!

     Moving onto LOW res, and again starting with the pixels:

                    P1 > P16;   P17 > P32;   and so on...

     For  this  case each pixel needs 4 bits(B),  namely a >  d,  and  thus  4 
bitplanes are needed as follows:

                                 All  make  up  P16
                    +-------------+-------------+-------------+
                    |             |             |             |
             B1a > B16a;   B1b > B16b;   B1c > B16c;   B1d > B16d;
            B17a > B32a;  B17b > B32b;  B17c > B32c;  B17d > B32d;  ...
            ~~~~~~~~~~~   ~~~~~~~~~~~   ~~~~~~~~~~~   ~~~~~~~~~~~
              Plane 1       Plane 2       Plane 3       Plane 4


     In MONO,  as there is only 1 bitplane (i.e. 1 bit/pixel), coding is a lot 
less complex as 'P1>P16; P17>P32' is simply represented by 'B1>B16; B17>B32'.


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