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From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: converting tail patches to head patches...
Message-ID: <9099@hoptoad.uucp>
Date: 28 Nov 89 06:24:46 GMT
References: <63359@tiger.oxy.edu>
Reply-To: tim@hoptoad.UUCP (Tim Maroney)
Organization: Eclectic Software, San Francisco
Lines: 57

In article <63359@tiger.oxy.edu> sonenbli@oxy.edu (Andrew D. Sonenblick) writes:
>      myPatch()
>      {
>      long  rtsAddr;
>      asm{
>	   Move.L    (SP)+,rtsAddr	   /* save return address in rtsAddr */
>	 }
>      doMyStuff();
>      asm{
>	   Move.L    rtsAddr,-(SP)
>	   Lea	     @1,A0
>	   Move.L    (A0),A0
>	   Jmp	     (A0)
>	 }
>      }
>
>      Ok, so, what's the ups with this?  (I have tried all sorts of things
>      similar to this--most just make my mac hang, some crash, none works.)

Almost certainly, it's the fact that you moved the stack pointer at the
wrong time and by the wrong amount.  This is the sort of code you
should always step through with a low-level debugger; it'll show you
what's happening very fast.  So anyway, allocating an automatic
variable causes that storage to be pushed on the stack.  So, you are
saving rtsAddr in rtsAddr and trashing it at the same time, since it's
probably the top of the stack.  There's no reason to save/restore
rtsAddr at all; it's right where you want it to be.  You are not taking
into account the A6 frame pointer, either.  In other words, lotsa
problems.  Here's my stab at it:

      myPatch()
      {
      doMyStuff();
      asm{
          unlk      a6
          Lea       @1,A0	; @1 is a dc.l that has the old trap stashed
          Move.L    (A0),A0
          Jmp       (A0)
        }
      }

This should work (but I haven't tested it and I make no guarantees).
If it's not clear, take a look at Figure 5 of Inside Mac, Chapter 4
(page IM I-97).  And if your have a problem, put a break on the trap
you're patching, using MacsBug, then step into it and watch what
happens....

Hope this works for you.  One more thing -- there are a lot of Mac
programmers outside the United States, many of whom read and post
to this newsgroup.  People shouldn't limit the distribution to usa
unless there's some pressing reason to do so, like distributing the
Mac source to a DES encrypter or something.
-- 
Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, tim@toad.com

"Mere opinion without supporting argument is no more than the American
 Bandstand school of literary evaluation."  -- Tom Maddox