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From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac.programmer
Subject: Stashing A5 (was Re: Help With VBL Tasks)
Message-ID: <27674@ucbvax.BERKELEY.EDU>
Date: 22 Jan 89 22:51:55 GMT
References: <8343@orstcs.CS.ORST.EDU> <27628@ucbvax.BERKELEY.EDU> <451@internal.Apple.COM> <6347@hoptoad.uucp>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
Lines: 62

In article <6347@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
_>However, this doesn't address the fact that you may need a different A5
_>from the one in CurrentA5, which was David's point.  This may happen in
_>interrupt driven code that is not switched by MultiFinder.  VBL tasks
_>only run when their application has the processor, but not so things
_>like socket listeners or custom interrupt handlers.  I still don't know
_>any way around this except for stashing a copy of your A5 in
_>code-relative space, which is explicitly forbidden (with good reason)
_>by Tech Note #2.

Be careful here. What is forbidden is modifying your code resources. There
is no problem creating a block of exectuable code at run time, that was
created with a nice constant built -in:

	MOVE.L	#Correct A5,-(sp)
	JSR	socketListener.
	RTS
you create this by saying:

typedef struct JumpBufA5{
	short	moveInstruction;
	long	a5Val;
	short	jsrInstruction;
	long	destination
	short	rtsInstrutcion;
}JumpBufA5, *JumpBufA5Ptr;

	listenerDispatcher = (JumpBufA5Ptr) NewPtr(sizeof(JmpBufA5));
	listenerDispatcher.moveInstruction = MOVEOPCODE;
	listenerDispatcher.a5Val = (long) CurrentA5;
	listenerDispatcher.jmpInstruction = JSROPCODE;
	listenerDispatcher.destination = (long) socketListerner;
	listenerDispatcher.rtsInsuction = RTSOPCODE;

and declare socket listener as:
/*
 */
pascal void socketListener(correctA5)Ptr correctA5;{
	Ptr oldA5;

	oldA5 = SetA5(coeectA5);
	...
	SetA5(oldA5);
}

You might think the above might lead you into trouble on a 68020 because
of the instruction cache. But, the instruction cache should never be
holding any address that NewPtr() is going to return, so the 68020 should
never get confused.  After all, the system reads code in from disk, and if
that isn't modifying memory that is about to be executed, I don't know
what is!

You might think you  could optimise the dispatcher's JSR, RTS to a JMP
instruction. Don't try it: you'll screw up the compiler's popping of
the arguments in socketListener.

The advantage of this trick is it lets you program the MMU to generate an
interrupt if anyone tries to write on one of your code segments.

--- David Phillip Oster            --"When we replace the mouse with a pen,
Arpa: oster@dewey.soe.berkeley.edu --3 button mouse fans will need saxophone
Uucp: {uwvax,decvax}!ucbvax!oster%dewey.soe.berkeley.edu --lessons." - Gasee