Date   : Sun, 05 Apr 2009 13:17:57 +0200
From   : anders.carlsson@... (Anders Carlsson)
Subject: A quick question about EPROM programming

Alex Taylor wrote:

> From memory (I read a book on the subject a while back), the C64 will
> just jump in where an expansion ROM is present (e.g. a cartridge)
> when certain expansion bus pins are grounded.

Almost. There are two ways to autostart C64 cartridges: they either
need to contain the string CBM80 at $8004 or a pointer at $A000 to
bypass Basic startup. The latter only makes sense on 16K cartridges.
If neither test succeeds, it starts up in Basic.

So it appears the only way to make a dual system utility ROM would be
to have the BBC headers at the beginning and 8K into it have a pointer
to where the C64 would start execution. I don't know exactly what
utility it would be, perhaps the software to some memory card reader
which can share a few higher order routines.

A bit of a moot point, but perhaps an interesting exercise to program
EPROMs that can be used in more than one kind of machine. :-)

Best regards

-- 
Anders Carlsson