The ability to define multiple text windows on the BBC computer, despite the fact that only one can be active at a time, is put to use in many programs.

When designing a program, it is difficult to visualise the exact extent of the window being defined. This can be overcome by changing the character used to fill the current window on execution of a CLS command.

This character is by default the space character &20, hence the 'clear' window.

Changing this to &FF, the block character, fills in the window. This character is stored in the VDU workspace in location &358.

Try typing in the following:

   ?&358=&FF
   VDU28,5,20,35,5
   CLS

The extent of the window is now clear to see. Note, though, that changing modes resets both the character and the text window to their default values.

David Abbott
Horsham, Sussex