Date   : Mon, 03 Feb 2014 10:10:59 +0000
From   : jgh@... (J.G.Harston)
Subject: Problems with BBC Micro

Chris Osborn wrote:
> Trying to figure out what might be wrong with my BBC Micro. When I go
> into Mode 0 and hit return a bunch, the screen freaks out and does
> this:
> http://i.imgur.com/qxrNCRQ.jpg

That looks very similar to what is displayed if you change the video 
"display start" address to the bottom of memory with VDU 
23;12;0;0;0;:VDU 23;13;0;0;0; except the low memory is being displayed 
at the bottom instead of the top. That suggests the "wrap around adder" 
is not working. There is a bit of circuitry that detects when the 
display has scrolled and would be displaying past &8000 and adds the 
appropriate offset to the screen address to continue from 
&3000/&4000/&58000/&6000 onwards instead.

IC36 and IC39 forms the adder, it is controlled by IC32 and IC3. You 
can try gently pushing them to make sure they are properly seated in 
their sockets.

Does the same thing happen in other screen modes? It won't happen in 
MODE 7 as it doesn't use the address adder, but if it's happening in 
MODE 0 then exactly the same thing should happen in MODE 1 and MODE 2 as 
they all use the same amount of screen memory.

-- 
J.G.Harston - jgh@...      - mdfs.net