DisplayScreen1()
{
..
..
}

DisplayScreen2()
{
..
..
}

DisplayScreen3()
{
..
..
}


RedKeys()
{
....
  if (Keypressed ())
    {
    if (Screen<3)
       Screen++;
    else
       Screen=1;
    }
...
}

Initialize ()
{
...
Screen=1;
...
}


main (void)
{
Initialize ();
for (;;)
  {
  ...
  ReadKeys ();
  if (Screen==1)  DisplayScreen1();
  if (Screen==2)  DisplayScreen2();
  if (Screen==3)  DisplayScreen3();
  ...
  }
}