Each display window may or may not have an associated alphanumerics memory
depending on the options of the CREATE/DISPLAY command.
The alphanumerics memory is attached to the bottom of the display window and
consists of 3 lines, the number of
characters per line depends on the width of the image display.
The same three different fonts which are used for the overlay channel can be
used for alphanumeric characters.
Commands associated with the alphanumeric memory are:
LABEL/DISPLAY - write a string into the alphanumeric memory
CLEAR/ALPHA - clear the alphanumeric display