BBC Model B - video help

From: Philip.Belben_at_powertech.co.uk <(Philip.Belben_at_powertech.co.uk)>
Date: Tue Feb 24 11:31:08 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.
Received on Tue Feb 24 1998 - 11:31:08 GMT