Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!nic.MR.NET!umn-cs!berlin!grg
From: grg@berlin.acss.umn.edu (George Gonzalez)
Newsgroups: comp.sys.mac.programmer
Subject: Re: How to do a LEGAL tail-patch
Summary: Another way to (maybe) do a tail patch
Message-ID: <293@berlin.acss.umn.edu>
Date: 16 Feb 89 15:56:43 GMT
References: <1272@ccnysci.UUCP> <6541@hoptoad.uucp>
Organization: U of M MicroGroup, Minneapolis
Lines: 24


I've thought about this problem too.  Here's a scheme that might work.

	RealTrapAddr := GetTrapAddress( TheTrap );
	SetTrapAddress( TheTrap, OurHookProc );
	PokeLong( @MyTailPatch, SingleStepInterruptVector );

	proc OurHookProc;	; gets called by the trap dispatcher.
	bset	#TrapBit,XXX(SP)	; set single step trap bit in ret flags
	PushL	RealReapAddress		; a registerless "jump"
	RTS

	... the trap code runs, then when it returns, the CPU does a single
	step trap to your tail patch:

	proc	MyTailPatch
	... do my post processing...
	bclr	#TrapBit,YYY(SP)	; turn off single stepping
	RTE


Besides the obvious problem of being able to patch only one trap at a time,
and needing different XXX and YYY offsets for 68000 and 68020+, this idea
*might* just work.