Path: utzoo!utgpu!attcan!uunet!husc6!rutgers!apple!lsr
From: lsr@Apple.COM (Larry Rosenstein)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multifinder: how do I patch GetNextEvent?
Message-ID: <18914@apple.Apple.COM>
Date: 17 Oct 88 16:04:56 GMT
References: <747@ttrdf.UUCP> <10050025@eecs.nwu.edu> <2456@spray.CalComp.COM> <7374@well.UUCP> <5654@hoptoad.uucp>
Organization: Advanced Technology Group, Apple Computer
Lines: 38

In article <5654@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>
>Let's look at a case.  Say that GetNamedResource checks the return address
>to see if it's being called from a ROM Font Manager routine, and that routine
>has a tail patch for some reason.  When the "old" software for the Font

The problem occurs when there is a tail-patch of GetNamedResource (for
example, in an INIT).  The system patches are installed at boot time, while
the INIT patches are installed afterwards.  

So now the Font Manager calls GetNamedResource which calls the INIT code.
The INIT code calls the original GetNamedResource via a JSR, which leaves
the return address in the INIT code.  This means that the system patch does
not recognize the return address, and the "bug" in the Font Manager is not
fixed.

This assumes that the INIT patch code exits by doing an RTS and returning
the result gotten by the JSR to the old GetNamedResource.  Under this
circumstance, the bug fix code would not be executed.

If it exits by calling the old GetNamedResource again (with the original
parameters), then everything should be fine.  The internal JSR simply
becomes another call to GetNamedResource.  (I don't write system patches,
however, so there may be something else I haven't thought of that makes this
idea wrong.)

Patching the ROM in a "safe" way requires some ingenuity.  Often ones first
idea of how to do it makes some incorrect assumption, or requires a tail
patch.  The trick is to look at the problem from a different point of view,
in order to come up with a solution that works.



		 Larry Rosenstein,  Object Specialist
 Apple Computer, Inc.  20525 Mariani Ave, MS 46-B  Cupertino, CA 95014
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