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From: jmunkki@kampi.hut.fi (Juri Munkki)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Re: Legal Tail Patches
Message-ID: <19886@santra.UUCP>
Date: 22 Feb 89 13:41:40 GMT
References: <1285@ccnysci.UUCP> <10050076@accuvax.nwu.edu>
Sender: news@santra.UUCP
Reply-To: jmunkki@kampi.hut.fi.UUCP (Juri Munkki)
Organization: Helsinki University of Technology, Finland
Lines: 31

In article <10050076@accuvax.nwu.edu> bob@accuvax.nwu.edu (Bob Hablutzel) writes:
>Are Apple people the only people blessed to do come-from patches?

Yes. I would never write a come-from patch even if I knew it was allowed.

The problem is that come-from patches prevent tail-patching. There is a
rather complicated way in which apple could allow tail-patching while
still doing their own come-from patching. Apple could change SetTrapAddr
aj GetTrapAddr so that the actual trap dispatch table is not changed for
the come-from patched traps, but the current patch is "told" that someone
wants to patch it. The come-from patch could then still do the come-from
stuff and it could then safely jsr/jmp to the patched routine. GetTrapAddr
should return the address that the come-from patch calls. This way no one
will loose, but programmers will be allowed to do tail patching.

To illustrate:
Before patching				After patching
_______________				______________
Program calls a trap			Program calls a trap
Trap dispatcher calls routine		Trap dispatcher calls routine
Come-from patch does something 		Come-from patch does something
Come-from patch calls ROM		Come-from patch calls new patch.
					New patch calls ROM...

Apple: Please implement this in the next system release...


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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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