Path: utzoo!mnetor!uunet!mcvax!ukc!mupsy!liv-cs!unpowell
From: unpowell@csvax.liv.ac.uk
Newsgroups: comp.sys.atari.st
Subject: Re: Video interrupts: using or disabling
Message-ID: <532@csvax.liv.ac.uk>
Date: 15 Mar 88 18:44:19 GMT
References: <3379@chinet.UUCP>
Lines: 30
Organisation: Computer Science CSVAX (VAX1), Liverpool University


In article <3379@chinet.UUCP>, saj@chinet.UUCP (Stephen Jacobs) writes:
> I am curious about the horizontal and vertical blank interrupts.  How much
> code can one safely put in a horizontal blank routine?  How many micro
> seconds does each call of the default vertical blank handler take?  There
> appears to be a flag telling the default vertical blank handler to cut its
> operation short: what awful thing (if any) would happen if the interrupt
> threshold was simply set above the vertical blank level?  And finally, how
> do I try playing with these things (I've never done anything with privileged
> instructions on the 68000, so those interrupt levels and first 1 K of memory
> flags are new territory).  I have Mark Williams C (with their assembler
> included), a few Abacus books and some 68000 documentation.  Use your best
> judgement about whether to mail or post replies.

	Better using Timer B (the MFP timer that counts line returns). This
gives much better results than the priority 2 HBL, because it has priority
6, and also has higher priority than keyboard interrupts etc.
	Even though it has higher priority than keyboard you may still have
to disable keyboard interrupts as there are about 50 clock periods taken
up with the imterrupt acknowledge. During this time no other interrupts
can be processed. This isn't much of a problem eith normal interrupts, but
with graphics this can create some awful flicker.

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 "...there's no success   JANET unpowell@uk.ac.lis.csvax
  like failure and        UUCP  {backbone}!mcvax!ukc!mupsy!lis-cs!unpowell
  failure's no success    ARPA  unpowell%csvax.lis.ac.uk@nss.cs.ucl.ac.uk
  at all..." B.Dylan

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