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From: jmunkki@kampi.hut.fi (Juri Munkki)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Keywords: Patches, Tails, "The Right Thing To Do"
Message-ID: <26596@santra.UUCP>
Date: 11 Nov 89 09:37:56 GMT
References: <1459@sequent.cs.qmc.ac.uk> <36250@apple.Apple.COM> <5056@internal.Apple.COM> <1989Nov7.212837.5146@oracle.com>
Sender: news@santra.UUCP
Reply-To: jmunkki@kampi.hut.fi (Juri Munkki)
Organization: Helsinki University of Technology, Finland
Lines: 31

Tail patching seems to be one of those subjects that pop up now and then.
Most people seem to agree that Apple is doing a stupid thing with the come
from patch method. In addition to making tail patching impossible, they make
the system software much more fragile.

About six months ago the net was fighting with Apple asking them to change
the way they install patches. I even came up with a method that in my humble
opinion would solve the whole problem from the point of view of developers
who want to tail patch.

The idea is to use a shadow dispatch table for those traps that are used
for come from patching. Calling GetTrapAddress gives that address from the
shadow table and calling SetTrapAddress changes the shadow dispatch table.
The addresses in the old dispatch table always point to routines that use
the come from method. The come from patch does it's processing and then
directly jumps into the shadow dispatch table.

The best thing would of course be to change the way apple patches those
long routines, but since they have started doing something stupid, it is
very unlikely that they will ever stop.

I wonder how much come from patching slows down the system. Most traps
that are used for come from processing are short utility traps. Perhaps
the most annoying examples are FixMul, FixDiv, FixRound and SectRect.
I recommend that you use your own versions of these routines, if you
intend to do any time-critical processing. 

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|     Juri Munkki jmunkki@hut.fi  jmunkki@fingate.bitnet        I Want   Ne   |
|     Helsinki University of Technology Computing Centre        My Own   XT   |
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