Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!apple!shebanow
From: shebanow@Apple.COM (Andrew Shebanow)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <5063@internal.Apple.COM>
Date: 6 Nov 89 18:51:25 GMT
References: <1459@sequent.cs.qmc.ac.uk>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 41

Tail patches are trap patchs which do processing
after calling the original trap, or modify the stack
before calling the original patch. They're called
tail patches because the typically look like this:

	LEA	origTrapAddress,A0
	; do preprocessing
	JSR	(A0)
	; do postprocessing

Since the bad part is at the end of the patch, its a
tail patch.

A "clean" patch will have a format like this:

	; do processing
	LEA	origTrapAddr,A0
	JMP	(A0)

Tail patching is bad because of the techniques that Apple
uses in its System Software to fix bugs. Sometimes, a trap
(lets call it _TrapA) has bugs in it which would be very
difficult to fix without rewriting the trap's entire code.
Rather than do that, Apple will sometimes patch a different
trap (_TrapB) which is called by the broken trap (_TrapA).
_TrapB will check the address of the caller on the stack
and compare it to the hardcoded address of _TrapA's call,
and, if the addresses match, behave in a different manner
to fix the bug. This can cause a huge savings in speed and
memory usage, but it prevents the use of tail patches.
Continuing our example, if you patch _TrapA and then JSR to
the original _TrapA code, you will have changed the
contents of the stack. And believe it or not, some of the
patches are involved enough that they look two or three
stack levels high.

Hope this clears it up,

Andy Shebanow
Mr. Clean, MacDTS
Apple Computer, Inc.