Date   : Mon, 02 Oct 2006 20:15:14 +0100
From   : Sprow <info@...>
Subject: Re: One more BBC B to fix

In article <672022E79B6349DBA63CA47178833796@...>,
   Andrew Chesterton <andrew.chesterton@...> wrote:

> > The RAM all feels the same temp. The 6502 and VIDPROC are 
> > warm too. Upon switching it on today, I got a flashing cursor 
> > in the top left corner in what looked like a mode 4 screen. 
> > Crtl-Break did nothing so I turned it off and back on again 
> > and it went back to the white screen with random LEDs.
>
> Somebody correct me if I'm wrong, but this random cursor behaviour could
> signify an addressing or Language issue (ie BASIC not working). If you are
> getting a cursor, then the OS has started and is looking for something to
> load, no cursor but random machine behaviour could be the machine has
> crashed 

The cursor flashes "for free" in hardware, so doesn't really hint whether
there is any software running - press and hold 'break' on a working machine
and you'll see the cursor keeps flashing. 

As it's a write only register and its reset state isn't mentioned in the
datasheet it may not be initialised to anything sensible at reset, so a
cursor being present at reset is possibly random.

> in the process of loading the OS and is now accessing the wrong
> locations in the memory map. 

The OS isn't loaded per se, it's just there. I agree with the general
suggestion that the address decode is a likely suspect, the OS needs RAM to
work everytime it uses a JSR/RTS pair,
Sprow.