Date : Thu, 22 Sep 1994 09:47:06 +0100
From : jfid@... (James Fidell)
Subject: Re: Start up in mode 0, you ****** thing...
"Chris L. Rae" wrote:
> However, I have a problem. If I clear *all* the BBC memory on bootup, my
> BBC attempts to go into mode 7; not much use. If I set all the VIA
> memory in Sheila to 1, it goes into mode 4 or something (one of the ones
> with &5800 start address) and reports "BBC Computer" (no memory,
> apparently!) then "Searching" as if it's jumped into the ROM FS.
I think that you've stumbled across the problems with not emulating Sheila
properly. You can't just stuff random values into the Sheila memory map
and expect it to work.
BTW, one of the possible reasons for the behaviour above is that with the
VIA set to all &01, the keyboard scan returns something that gets interpreted
as a DIP switch setting for MODE 4, the OS doesn't do a hard reset because
it doesn't believe that the VIA is in it's power-up state, so you don't
get it checking how much memory there is and it probably thinks SHIFT
is pressed, so it starts searching (on tape ?) for a file to boot.
> A) How can I get it to think it's in mode zero
You need to properly emulate the keyboard so that when the OS scans the
location at which the DIP switches are mapped into the keyboard array, it
will get the setting for the start-up mode. Once you've done that, you
can make the settings of the DIP switches a command line option.
> B) Why does it not report 16/32K?
Probably because the OS scans the system VIA to decide when it's been
powered up or just hardware reset. You need to have the VIA set up
correctly to get the OS to decide it's going to wipe the contents of
RAM and work out how much memory there is. Of course, the other way
to get the OS to check memory is to do CTRL-BREAK, but that's kind
of difficult without the keyboard too.
James.
--
"Yield to temptation -- |
it may not pass your way again" | jfid@...
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- Lazarus Long | James Fidell