BBC Model B - video help - test-drb@ccmp.vtda.org

24 Feb 1998

Tony Duell quoted me as having written:

...
   On the subject
of BBC video problems, it occurs to me that the BBC micro 
 does scrolling by moving the pointer to the start of the screen (under 
 some conditions?).  If you can get it to do this, and see how the 
 display behaves, you may be able to determine easily if it's an 
 addressing problem. 
 Is this going to tell you very much?

 The BBC _does_ use hardware scrolling (it changes the start-of-memory 
 register in the 6845), but as the screen is a contiguous array of bytes, 
 all that a scroll does is effectively increment that pointer by the 
 number of bytes/line of characters (= 640 bytes in mode 0, etc) and then 
 clear the new bytes displayed on the bottom line.

 So the new line 0 will be the old line 1, etc, but there are no other 
 changes. A given byte on the screen is displayed in the same relative 
 position to other given bytes on the screen. In mode 0, &3281 is still the 
 second dot row of a character in the far left column. Of course if the 
 text goes all over the place on a hardware scroll then you can be 
 virtually certain that the 6845 is playing up.  
What I meant was that addressing problem _external_ to 6845 generally 
means that the fault will occur at the same _memory_ address.  So that 
type of scrolling means the fault will scroll up the screen, appearing 
at the same place in the _text_ every time.  Won't it?

If the screen line length were an integer power of two, a fault in the 
bottom few address lines would appear at the same place on every line.  
Fortunately the BBC micro is 80 characters per line, so you will see 
things recurring on a slant.  This slanted line of fault should move up 
the screen as it scrolls.

Something to try, anyway.  I am a firm believer in non-invasive tests!

Philip.