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Message: 2238

Author: andrewbroad

Date: 14/05/2001

Subject: Re: invalid arrows / the Attic Bug

 

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/
http://www.cs.man.ac.uk/~broada/spectrum/
http://www.cs.man.ac.uk/~broada/spectrum/willy/
http://www.cs.man.ac.uk/~broada/spectrum/willy/bugs.html
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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