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From: holland@mips.csc.ti.com (Fred Hollander)
Newsgroups: comp.sys.mac.programmer
Subject: Re: How to Patch
Summary: example source
Keywords: trap patch init
Message-ID: <63003@ti-csl.CSNET>
Date: 8 Nov 88 16:43:54 GMT
References: <10050042@eecs.nwu.edu> <563@poseidon.ATT.COM>
Sender: news@ti-csl.CSNET
Reply-To: holland@mips.UUCP (Fred Hollander)
Organization: TI Computer Science Center, Dallas
Lines: 67

Thanks to those who responded to my request for help.  I finally got
something to work.  Here it is in case anybody else would like to use it.
Build a code resource, type INIT, with the System and Locked attributes.
There's a bit of a kludge in there because, unless I'm mistaken, the
standard header provided by LightSpeed C trashes A0 beyond recovery.  I
modified the header (in ResEdit) by putting an extra instruction to push
registers A0-A2 on the stack.  An alternative would be to store the globals'
base address and skip the header or write a custom header.  Does anyone have
the standard header?  What else does it do besides put the entry point into
A0?

Fred Hollander
Computer Science Center
Texas Instruments, Inc.
holland%ti-csl@csnet-rela

The above statements are my own and not representative of Texas Instruments.
The code below is my own and not representative of Texas Instruments.

#include <SetUpA4.h>

#define	trapSysBeep		0xA9C8

long	oldTrapAddress;
Handle	trapHandle;
char	installed = false;

void main () {
	/*  
	Manually put this (48E7 00E0) in the header at 0x000C
	so that it is called before AO is trashed!
	Change 0x0000 from 600E to 600A to point to this move.
	(May be easier to write a custom header.)
	asm{
		movem.l		A0-A2,-(A7)
	}
	*/
	RememberA0 ();
	SetUpA4 ();
	if (installed) {
		/* Put your stuff here */
		asm{
			move.l		oldTrapAddress,A3
		}
		RestoreA4 ();
		asm{
			movem.l		(A7)+,A0-A2
		}
		asm{
			jmp		(A3)
		}
	} else {
		asm{
			move.l		A4,A0
			_RecoverHandle
			move.l		A0,trapHandle
		}
		DetachResource (trapHandle);
		HLock (trapHandle);
		oldTrapAddress = GetTrapAddress (trapSysBeep);
		SetTrapAddress ((long) *trapHandle, trapSysBeep);
		installed = true;
		RestoreA4 ();
		asm{
			movem.l		(A7)+,A0-A2
		}
	}
}