Date   : Tue, 30 Aug 1994 09:12:30 WET DST
From   : Bonfield James <jkb@...>
Subject: Re: Emulator Fundamentals

To expand upon Stephen Quan writings:

>When the 6502 is done, you can load in the OS rom at $C000-$FFFF,
>and load in a memory image of $000-$DFF and set your PC to the
>word address stored in $FFFC, and you things will work.

Also, the memory dump of 000-dff I produced by a simple *SAVE on my beeb. It's
not required really, but aids startup procedures when you've got no 6522
support done. I found that once the 6522 is working the dump may cause
problems and so should then be thrown away. It's just a cheat (like the OSRDCH
and OSWRCH entry point hacks) that allows testing of the first parts of the
emulator before completing the final system.

Also, the OS ROM isn't actually &4000 long. Well, it probably is on the chip
itself, but the top &FC00 - &FFFF isn't mapped into the BBC memory. These top
pages are Fred, Jim and Sheila. Sheila in particularly is complex. It contains
many memory mapped support chips. It may help to initialise this to some
sensible default (I've heard people say that 0x80 is good) or (as I did)
initially fill it with sensible defaults obtained by copying the memory down
to 'user' (0000-7fff) memory and *SAVEing it.

>If all works, you should find the system boots up
>but reports that there is no language (ie. BASIC).  But you'll
>be able to do simple * commands.

Is this true of the BBC? I thought that's only correct on Masters. I thought
that with the BBC things simply hang without BASIC. Infact I don't think you
can boot up correct even with another language present, but no BASIC.

       James