Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!cs.utexas.edu!usc!apple!tecot
From: tecot@Apple.COM (Ed Tecot)
Newsgroups: comp.sys.mac.programmer
Subject: Re: converting tail patches to head patches...
Message-ID: <36872@apple.Apple.COM>
Date: 30 Nov 89 03:44:01 GMT
References: <63359@tiger.oxy.edu> <5437@internal.Apple.COM>
Distribution: usa
Organization: Apple Computer Inc, Cupertino, CA
Lines: 31

In article <5437@internal.Apple.COM> chewy@apple.com (Paul Snively) writes:
>Now for the good news: if you're trying to patch _SystemTask, you can 
>probably get away with it, because I don't believe that _SystemTask is 
>called from within the ROM, which means that it won't be patched by us 
>with this nasty come-from code that's causing all of the headaches.  As 
>Larry Rosenstein pointed out, you have a higher probability of being 
>"patch safe" with traps that have a high probability of not being called 
>from ROM.

Actually, Paul is wrong.  SystemTask is called from ROM, in particular, by
ModalDialog and WaitNextEvent.  It's possible that it is called in other
places as well.  However, it is called in relatively few places (compared
to NewHandle or GetResource), and is therefore safer to tail patch.  Note,
that it is not 100% safe, and in this case, it's relatively easy to convert
the tail patch to a head patch, since SystemTask doesn't return anything.
Use

MySystemTask
	(your patch goes here)
	JMP	OldSystemTask

instead of

MySystemTask
	JSR	OldSystemTask
	(patch code here)
	RTS

and your code is 2 bytes shorter and 24 cycles faster as well.

						_emt