Translating BBC physical colours to ANSI colours
                 ================================================

When implementing a VDU interface between BBC BASIC and an OS host, this is the
mapping that should be used if the host uses ANSI colour selection.

Physical colours
----------------
In display modes with 8 or more colours, the physical colours should be the closest
match to RGBIF numbering. These are the colours that should be used on systems with
no colour palette, or with the default palette selected where 8 or more colours are
available. Displays with 256 or more colours and teletext displays use a different
scheme not covered here.

Physical colour numbers, the parameter to COLOUR and VDU 17,n should be:
  +-+-+-+-+-+-+-+-+
  |x|x|E|F|I|B|G|R|
  +-+-+-+-+-+-+-+-+
   | | | | | | | +-Red
   | | | | | | +---Green
   | | | | | +-----Blue
   | | | | +-------Intensity
   | | | +---------Flash
   | | +-----------Extension
   0 0-------------Select foreground colour
   0 1-------------Extension
   1 0-------------Select background colour
   1 1-------------Select border colour where supported

Extensions are implementation specific. For example, the ZX Spectrum uses bits 5 and
bit 6 to independently select Flash, Bright and RGB.

Translation
-----------
The translation from physical colour numbers to ANSI colour sequences is as in the
following C code.

  ansi_colour(c) {                              // c = colour parameter
      char ansichars[]="\x1B[25;24;27;21;30m";
      char *seq;

      if ((c & 0xC0) == 0xC0) return;           // &C0+n, set border, ignore
      seq=&ansichars[2];                        // CHR$27;"["
      if ((c & 0x10) == 0) *seq++='2';
      *seq++='5'; *seq++=';';                   // Flash
      if ((c & 0x20) == 0) *seq++='2';
      *seq++='4'; *seq++=';';                   // Underline
      if ((c & 0x40) == 0) *seq++='2';
      *seq++='7'; *seq++=';';                   // Inverse
      if (c & 0x80) {
        *seq++='4'; *seq++='0'+(c & 7);         // Background colour '40'-'47'
        if (c & 0x08) { *seq++=';';
                        *seq++='1';
                        *seq++='0'; 
                        *seq++='0'+(c & 7);     // Bright background '100'-'107'
        }
      } else {
        if ((c & 0x08) == 0) *seq++='2';
        *seq++='1'; *seq++=';';                 // Bright foreground
        *seq++='3'; *seq++='0'+(c & 7);         // Foreground colour '30'-'37'
      }
      *seq++='m'; *seq++=0;
      fputs(ansichars,stdout);                  // Send ANSI sequence to display
  }

This will build a string with the following components:
    CHR$27+"[25;24;27;21;30m"
 or CHR$27+"[25;24;40;100m"

for example:
    CHR$27+"[25;24;27;21;34m"   COLOUR 4   - blue foreground
    CHR$27+"[25;24;27;1;37m"    COLOUR 15  - bright white foreground
    CHR$27+"[5;24;27;21;33m"    COLOUR 19  - flashing yellow foreground
    CHR$27+"[5;24;27;1;31m"     COLOUR 25  - bright flashing red foreground

    CHR$27+"[25;24;40m"         COLOUR 128 - black background
    CHR$27+"[25;24;44m"         COLOUR 132 - blue background
    CHR$27+"[25;24;43;103m"     COLOUR 139 - bright yellow background

Different ANSI interpreters select bright colours differently. Some allow both
foreground and background brightness to be selected, some set the foreground and
background to the same brightness, some only set the foreground brightness. Some only
recognise '40'+colour, some recognise '100'+colour, so you should emit both sequences
when setting a bright background.

Many ANSI interpreters do not implement 'flash', and interpret it as 'highlight'
setting a bright background instead.