Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!unido!mpirbn!p554mve
From: p554mve@mpirbn.UUCP (Michael van Elst)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Reading absolute memory locations
Message-ID: <546@mpirbn.UUCP>
Date: 8 Feb 90 15:38:26 GMT
References: <75.25cde84e@intersil.uucp>
Reply-To: p554mve@mpirbn.UUCP (Michael van Elst)
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn
Lines: 23

In article <75.25cde84e@intersil.uucp> hamilton@intersil.uucp (Fred Hamilton) writes:
>Hello World.
Hello Human :-)

>if I've already blown it).  So I wrote a program to read some addresses
>in the $00D80000-$00D80044 range (the same range the 500/2000's clock 
>resides at), and I found I got this bizzare screen glitch when I read 
>location $00D80028.

The stock Amiga maps the custom chip registers into addresses $c00000-$dfffff.
They are mirrored some hundred times and so (when you read at $d80028) you'll
actually read $dff028 which seems to be a WRITEONLY register. A WRITEONLY
register will even write something into itself if you read from it's
address. Since the 68000 wouldn't drive the data bus, you'll get some
random number written into that register.
If you enable GENLOC mode this way, you'll shutoff the main system clock
(and enabling an external clock). This will harm processor and memory
refresh functions.

Ugh, (I've experienced it myself).

Michael van Elst
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