MY>just write a simple program to display, say, a blue letter 'A' in
 MY>the fifth position of video memory, I think I could take it from
 MY>there.

  #define BLUE            1
  #define cht(c,a)        (((c) & 255) | ((a) << 8))

  USHORT cell = cht('A',BLUE);

  VioWrtCellStr(&cell,1,0,4,0);  /* not direct video memory access */

You're going to need an API reference (and you're going to need to use
it).  You'll also need to break the DOS habit of writing single chars --
write blocks instead:

  VioWrtCellStr("Holy video input/output subsystem, Batman!",42,0,0,0);

--- XHEd-OS/2 1.22