KeyScan()

         Extracts a character from the keyboard buffer

    Syntax:

         KeyScan() -> nKey

    Description:

         Extracts a character from the keyboard buffer or halt the program
         until a key is pressed.

         KeyScan() returns an integer value identifying the key extracted
         from the keyboard buffer.

                nkey % 256 = ascii code   (0 in function keys)
                nKey / 256 = scan code

    Return:

         The key pressed.

    Example:

         nKey := KeyScan()
         ? nKey / 256
         ? nKey % 256