Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!cmcl2!ccnysci!alexis
From: alexis@ccnysci.UUCP (Alexis Rosen)
Newsgroups: comp.sys.mac.programmer
Subject: How to do a LEGAL tail-patch
Message-ID: <1272@ccnysci.UUCP>
Date: 15 Feb 89 08:07:41 GMT
Reply-To: alexis@ccnysci.UUCP (Alexis Rosen)
Organization: City College of New York
Lines: 19


I've been thinking about this for a couple of days now. Here's my scheme.
Does anyone see any flaws?

Procedure MyPatch
SetTrapAddress to original trap address
Trap to original trap using A-line instruction
SetTrapAddress back to @MyPatch (&MyPatch for C people)
Fixup stack and return

It seems very simple. The only trick is that you have to store the original
trap address in code-relative space (stick it after the end of the patch code).
This is not a difficult thing to do, though it will break in a virtual-memory
system (by that point there will have to be legitimate ways to do this anyway).

So, what's the verdict?

Alexis Rosen
alexis@ccnysci.uucp