Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!apple!apple.com!chewy
From: chewy@apple.com (Paul Snively)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <5080@internal.Apple.COM>
Date: 8 Nov 89 17:34:28 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 36
References:<1459@sequent.cs.qmc.ac.uk> <36250@apple.Apple.COM> <5056@internal.Apple.COM> <1989Nov7.212837.5146@oracle.com>

In article <1989Nov7.212837.5146@oracle.com> omullarn@oracle.oracle.com 
(Oliver Mullarney) writes:
> OK - I understand why we shouldn't use tail patches, but sometimes
> you just gotta. How else can you catch a function result from a
> trap call?
> Now why doesn't Apple keep everyone happy by being a little bit
> more inventive in their scheme for hacking patches called by traps
> called by patches called by ...
> Instead of maintaining hard coded return addresses (a bad move anyway,
> considering TN 110, "Processor Compatibility", final para.) it would
> be possible to examine the trap table for the address of the trap of
> interest (in case it has been patched), scan forward from this address 
> to find the RTS, and check to see if the return address on the stack 
> lies between the trap/patch start and the RTS. Not lightning fast, but...
> Maybe in the 50Hz machine we wouldn't even notice.

As for "sometimes you just gotta," all I can say is: don't do whatever it 
is you're doing that's "just gotta."  Find a "more inventive" way.

As for finding the trap address and looking "just within that trap" for 
the return address, I can only assume that you've never disassembled our 
ROMs.  Essentially what you're saying is that we should write a 680x0 
global flow-analysis routine that gets called for EVERY patch in the 
system.  Even at 50 MHz that would be UNBELIEVABLY slow, and that's 
assuming that your flow-analysis heuristic for 680x0 code was totally 
flawless.  Anyone here ever write a 680x0 flow-analysis program (Steve 
Jasik excepted)?  Didn't think so.

Don't tail patch.

__________________________________________________________________________
Just because I work for Apple Computer, Inc. doesn't mean that they 
believe what I believe or vice-versa.
__________________________________________________________________________
C++ -- The language in which only friends can access your private members.
__________________________________________________________________________