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From: dac@ukc.ac.uk (David Clear)
Newsgroups: comp.sys.atari.st.tech
Subject: Re: Using Timer A to replay a sample
Message-ID: <5558@harrier.ukc.ac.uk>
Date: 28 Sep 90 08:16:05 GMT
References: <1990Sep24.122920.15704@irscscm.UUCP> <891@ehviea.ine.philips.nl> <892@ehviea.ine.philips.nl>
Reply-To: dac@ukc.ac.uk (David Clear)
Organization: Computing Lab, University of Kent at Canterbury, UK.
Lines: 41

In article <892@ehviea.ine.philips.nl> leo@ehviea.UUCP (Leo de Wit) writes:
>
>The sequence for timer A is (I looked it up):
>
>my_timint
>    ... do your stuff here ...
>    bclr #5,$fffa0f
>    rte
>

This sequence is correct - I have had no problems using it. I've never
seen any documents to suggest that 1s have to be written into the other
bits of the "interrupt in service" register. As I understand it, when
the MFP gets an interrupt, it will not signal the 68000 unless it is
of a higher priority than those already in service. Similarly, if it
does signal the 68000, the 68000 will not service it unless the IPL has
been moved back below 6 (?).

So, for general purpose, multi-level interrupts, the order of events
should be:

int:	move.w	#$2300,sr	/* Also enables VBI, #$2400, disables VBI */
	.. your own thang
	bclr.b	#n,MFP_ISR[AB]
	rte

On the subject of sampled sound drivers, as I understand it, to plug a
value in a volume register you first do a register select and then you
plug the data in. This can be done with a movep.w, right? Wanting to speed
up his sample player, a friend of mine discovered that the sound chip only
decoded the lowest bit of the address - so the CONTROL/DATA register pair
were replicated throughout the address space of the chip. This means that
rather than doing three movep.w instructions, you can get away with
a movep.l and a movep.w. Of course, this can't be guaranteed to work.

Dave.
-- 
% cc life.c                      | David Clear dac@ukc.ac.uk +44 227 764000x7592
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Segmentation fault (core dumped) | University of Kent, Canterbury, England.
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