Date   : Mon, 16 Apr 2012 15:11:48 +0100 (WET-DST)
From   : BBCMICRO@... (Peter Coghlan)
Subject: MODE 7 video corruption

>
>> Another thought: is it possible that the lines you are seeing are 
>> cursors
>> rather than displayable characters?
>
>Looking at the video it does look like a cursor flickering all over
>the screen. It's a consistant MODE 7 cursor width and is aligned
>with the character spacing.
>

Do they appear below the character positions as cursors would or half way
up the character positions as long dashes would?

Does this program produce a similar effect on a correctly working Beeb?:

10 MODE 7
20 PRINT "BBC Computer"''"BASIC"
30 REPEAT VDU 31, RND(39), RND(24)
40 UNTIL FALSE

Does turning off the cursor with:

VDU 23, 0, 10, 32, 0, 0, 0, 0, 0, 0

eliminate the effect on the faulty beeb?

If they are cursors, forget anything else I've said before now!
Interesting things to check would be whether the correct behaviour occurs when
an arrow key is pressed to cause the cursors to split and whether the cursors
behave correctly in other modes.

Assuming they are cursors, the next place to look is probably at the 6845 (IC2)
and the Vidproc (IC6) which between them seem to be responsible for cursor
genration and positioning. Registers R14 and R15 in the 6845 contain the
cursor position which seems pertinent here and unlike most of the rest of the
registers, are readable as well as writable so we can check what values are in
them.

Regards,
Peter Coghlan.