Date   : Tue, 19 May 2009 11:48:21 +0100
From   : michael.firth@... (michael.firth@...)
Subject: MOS3.5/3.2 switchable modules and Y2k

> -----Original Message-----
> From: 
> bbc-micro-bounces+michael.firth=bt.com@... 
> [mailto:bbc-micro-bounces+michael.firth=bt.com@...
> .uk] On Behalf Of me@...
> Sent: 19 May 2009 11:32
> To: BBC micro mailing list
> Subject: Re: [BBC-Micro] MOS3.5/3.2 switchable modules and Y2k
> 
> On 18 May 2009, at 16:29, Mark Haysman wrote:
> 
> > Also, if you need a "custom" build of the DualOS, just send me a  
> > mail, the
> > main chip is just a 2Mbit Flash you can program yourself
> 
> 
> So what happens if you put OS 1.20 in it? Does that work on Master  
> hardware? Would that solve compatibility problems with old BBC games?
> 
I finally got around to trying that on my real Master a couple of weeks ago.

Basically, when it powers up correctly, it boots as OS 1.2, and is able to access
a 1MHz bus connected CF card (or presumably a legacy ST506 HD), and I expect that
cassette access should work too.

Floppy disk access is a bit more work, which I haven't found time to sort
out yet,
as a special version of the FS will be needed, as the FDC hardware is at
different
addresses in the Master than the B/B+.

The bigger issue is that the machine only powers up correctly about 50% of
the time -
my suspicion is that this is because the 'ACCON' register isn't being initialised
by
the 1.2 code, so is powering up in a random state.

If my guess is correct, then the problem could be fixed by finding 6 free
bytes somewhere
in the 1.2 OS ROM to have
STZ &FE34
JMP &D9CD

placed in them, and then make the actual reset vector point to the new code.

Again, I haven't had time to hunt down where 6 spare bytes might be to try this.

Regards

Michael