Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!julius.cs.uiuc.edu!apple!gateway.qm.apple.com!sean_parent.snarkmail_l_z
From: sean_parent.snarkmail_l_z@gateway.qm.apple.com (Sean Parent)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Need jGNEFilter help
Message-ID: <11380@goofy.Apple.COM>
Date: 29 Nov 90 18:42:23 GMT
References: <917@gagme.chi.il.us>
Sender: usenet@Apple.COM
Distribution: na
Organization: Apple Computer, Inc.
Lines: 34

The problem with your patch is that it is a tail patch. (A tail patch is a 
patch that does work after calling the original routine instead of 
returning directly to the original caller.) This is breaking a come-from 
patch that MultiFinder has installed. (A come from patch is a patch that 
checks the return address to fix a routine in ROM.) There are a few 
possible options that you could use ->

1) Have your INIT install a driver to get time cleanly.
2) Patch SystemTask (I think it is a better place to patch then 
GetNextEvent and contrary to inside Mac it can move memory (because it can 
call drivers and desk accessories) using a head patch.
3) Patch GNE using a head patch.

When I patch traps I usually use asm code of the form ->

SUBQ.W   #4,SP           ; save space to chain
LINK     A6,#stackSpace  ; set up A6 frame
MOVEM.L  savedRegs,-(SP) ; save away ALL used registers (include scratch)

                         ; Do my work

MOVE.L   oldRoutineAddress,4(A6) ; put old address in chain space
UNLK     A6              ; restore A6 frame
MOVEM.L  (SP)+,savedRegs ; restore ALL registers
RTS                      ; chain to old routine

In this way, any come from patchs that depend on register values (yes, they 
do exist) are kept intact. Also, some ROM routines depend on some of the 
traps not messing with scratch registers. (For example, the Dialog Manager 
relies on the fact that SetPort only trashes A0 and non of the other 
scratch registers).

Sean Parent
"Quality unattainable in a reasonable amount of time."