Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!mintaka!bloom-beacon!eru!hagbard!sunic!chalmers.se!etek.chalmers.se!fy.chalmers.se!f90angu
From: f90angu@fy.chalmers.se (Andreas Gunnarsson)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: CIA register
Message-ID: <1991May17.143302.5828@fy.chalmers.se>
Date: 17 May 91 14:33:02 GMT
References: <1991May16.103515.30814@kuhub.cc.ukans.edu>
Organization: Chalmers University of Technology, G|teborg, Sweden
Lines: 35

In article <1991May16.103515.30814@kuhub.cc.ukans.edu> markv@kuhub.cc.ukans.edu writes:
[...]
> 
>Yes, the overlay bit maps the ROMs to 0.  That's ALL it does.
[...]
>However, this trick can be used to look at the boot ROMs in
>a 3000 or 1000 (the 1000 requires other magic too, because of the WCS),
>since its the *real* ROM that gets mapped in, not the ROM image.
>
The overlay bit maps the ROM at $f80000 to 0. In the 500 (and I'm sure the
other 256k ROM Amigas, though I haven't checked) $f80000-$fbffff comtains a
copy of $fc0000-$ffffff. However, in the A1000 the boot ROM is located at
$f80000.

To look at the boot ROM in a 1000 you simply have to jump to the RESET
instruction at the beginning of WCS ($fc0000). This will reset the computer,
and after that the ROM will be visible at $f80000 and the WCS is RAM! Now just
use your favorite monitor/debugger to (look at the boot ROM)/(change the WCS).
Of course, if you change anything you may crash the computer because it's
still running the kickstart from $fc0000.
When you write something to the address range $f80000-$fbffff, the boot ROM
will disappear and the WCS will become write protected. The boot ROM normally
does this, but since you've told the computer to bypass it and jump directly
to WCS, that instruction will not be executed.

DISC LAMER: This works on _MY_ A1000. Don't yell at me if it doesn't work on
            your!

==============================================================================
73 es 88 de SM7TLS          f90angu@fy.chalmers.se          Andreas Gunnarsson
==============================================================================
-- 
==============================================================================
73 es 88 de SM7TLS          f90angu@fy.chalmers.se          Andreas Gunnarsson
==============================================================================