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From: dan@lclark.UUCP (Dan Revel)
Newsgroups: comp.sys.mac.programmer
Subject: Re: MIDI Manager/Think C/A5 Register
Summary: saving A5 for completion routines
Message-ID: <915@lclark.UUCP>
Date: 13 Apr 90 22:32:04 GMT
References: <9004121500.AA00697@canoe.cis.ohio-state.edu>
Reply-To: dan@lclark.UUCP (Dan Revel)
Organization: Lewis & Clark College, Portland OR
Lines: 63

In article <9004121500.AA00697@canoe.cis.ohio-state.edu> james_kent@CIS.OHIO-STATE.EDU writes:
>I've been trying to use MIDI manager with Think C and have come across the
>following problem (at least as I understand it).
...
>I need a method of doing the following, using Think C :
>
>1) save the application's A5 value somewhere when the application starts
>        (Location X)
>2) when the interrupt routine is called, save the current A5 value (Location Y)
>    and move the value from Location X into A5
>3) when the interrupt routine completes, move the value from Location Y back
>    into A5 before returning

Your call-back routine should be of the form:

PROCEDURE MyCallBack(chan: SndChannelPtr; cmd: SndCommand);

[IM V-480] in C that would be:

pascal void MyCallBack(SndChannelPtr chan, SndCommand cmd);

Store your CurrentA5 either in the userInfo field of your SndChannel or
else put it just before your SndChannel and use Think C's assembler to
recover it, e.g.:

Ptr		savedA5;
SndChannel	myChannel;

mySetUp()
{
	/* what have you */

	savedA5 = CurrentA5;
}

pascal void MyCallBack(SndChannelPtr chan, SndCommand cmd)
{
	Ptr	myA5, oldA5;

	asm {
		move.l	-4(chan), myA5	; get savedA5
	}
	oldA5 = SetA5(myA5);

	/* do your stuff */

	SetA5(oldA5);	/* restore A5 for the interrupt when you are done */
}

pascal long SetA5(long newA5) = { 0x2f4d, 0x0004, 0x2a5f };

SetA5 is described in Tech Note #208 as 'The New, Totally Cool Way' to
do this instead of using SetupA5() and RestoreA5().

Tech Note #180 might also be of interest to you.

That's how you do it...  I don't make any promises about the accuracy
of my code, it is untested, I just wrote it as an example...

Good Luck.
-- 
dan@lclark
tektronix!reed!lclark!dan			Dylsexics untie! (-|