Date   : Thu, 28 Jul 1994 09:37:31 EST
From   : Stephen Quan <quan@...>
Subject: Video display.

Hi everyone!  I tested refresh rates on my BBC model B, and found : 

    * modes 0..6 adhere to following rule of thumb, (despite differing memory).
    * rule of thumb is 1 scan line per 128 cycles.
        (or 1 scan line per 64 NOP instructions)
    * nb. different modes have different total # of scan lines.
    * nb. different modes have different total bytes/scan line.

The scanline/128 cycles is not perfectly straight, ie. not

     ...........................................................

but
     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .
     : .': .': .': .': .': .': .': .': .': .': .': .': .': .': .':
     :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :'  :

Jaggies seems no heigher than 4 pixels and are restricted to one byte
of graphics.  ie. effect is worse in mode 0, but almost negligible in
mode 2.  I will be neglecting the effect of the jaggies myself.

The following C fragment captures roughly how I might implement this.

     if (cycles >= 128)
     {
       cycles -= 128;
       switch (mode)
       {
         0: draw_mode0_scan_line;    break;
         1: draw_mode1_scan_line;    break;
         2: draw_mode2_scan_line;    break;
         3: draw_mode3_scan_line;    break;
         4: draw_mode4_scan_line;    break;
         5: draw_mode5_scan_line;    break;
         6: draw_mode6_scan_line;    break;
         7: draw_mode7;              break;
       }
     }

Has anyone got a copy of that hack program that puts you into an
MODE 8?  I recall reading from a beeb magazine that you can take
MODE 5, but manipulate it so that it has 16 colours with no cost
in memory, but an obvious reduction in resolution.
- 
Stephen