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using Manic Miner and Jet Set Willy game engines
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Message: 2238
Author: andrewbroad
Date: 14/05/2001
Subject: Re: invalid arrows / the Attic Bug
On 10th January 2001, sendy_baby wrote:
>
> noticing with a certain glee
that the new jswed
> allows invalid arrows, i'm wondering if now's
> the time
to start questioning their effects.
>
> the main question on my mind is
this:
>
> if an arrow writes to the screen, can i be sure
> that it is not corrupting
other parts of the
> memory, a la the attic bug?
You could test for memory
corruption by comparing snapshots before and after an invalid arrow has been fired.
The
Attic bug is actually caused by an invalid arrow - its vertical position is off the screen,
which is why it corrupts memory (in particular, it writes to the Guardian Class Table
instead of video RAM as arrows are supposed to).
However, the invalid arrows we're
using are not off the screen, but straddle character row boundaries. As a result, they write
to the colour-attribute part of video RAM (22528-23295) rather than the pixel part
(16384-22527), so the possibility that they might overwrite other parts of memory is a valid concern.
I haven't noticed any other side-effects, though! ;-)
"Filsoft: Making Andrew
Broad reconsider what should be considered valid" (Philip Bee)
--
Andrew
Broad
http://www.cs.man.ac.uk/~broada/ target=new>http://www.cs.man.ac.uk/~broada/>
http://www.cs.man.ac.uk/~broada/spectrum/ target=new>http://www.cs.man.ac.uk/~broada/spectrum/>
http://www.cs.man.ac.uk/~broada/spectrum/willy/ target=new>http://www.cs.man.ac.uk/~broada/spectrum/willy/>
http://www.cs.man.ac.uk/~broada/spectrum/willy/bugs.html target=new>http://www.cs.man.ac.uk/~broada/spectrum/willy/bugs.html>
http://www.cs.man.ac.uk/~broada/spectrum/willy/bugs.html#attic_bug target=new>http://www.cs.man.ac.uk/~broada/spectrum/willy/bugs.html#attic_bug>
P.S. Yahoo! won't let me access Message 2236 at all, no matter which direction I come at it
from. Is anyone else having this problem? Big cracks are starting to appear in Yahoo! Clubs.
Perhaps it's a sign that the conversion of Yahoo! Clubs to Yahoo! Groups is imminent...
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