Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!tektronix!tekig!tekig5!wayneck
From: wayneck@tekig5.TEK.COM (Wayne Knapp)
Newsgroups: comp.sys.atari.st
Subject: Re: Video interrupts: using or disabling
Message-ID: <2509@tekig5.TEK.COM>
Date: 9 Mar 88 23:48:47 GMT
References: <3379@chinet.UUCP>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 35
Keywords: interrupt vector priority speed

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

 Horizontal blank is hard to use and the interrupt occurs every 64.5 usec.

> seconds does each call of the default vertical blank handler take?  There

 No idea, but there is time for lots of code during a vertical blank, at
 least a millisecond's worth of code.

> 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

  Ugh! You would very quickly get a stack overflow, crash, burn, ugly stuff.

> 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.


   When playing around with the interrupts you will get many surprizes.  
Video interrupts can be very trickly.  If you only have a small routine I
run just add it to the vertical blank routine.  I spent a lot of time playing 
with the horizontal interrupts and finally gave up.  Just too much overhead
to try and handle a iterrupt every 64.5 usec.  Doing stuff with video
interrupt can give great results but it is very easy to burn up most of the
cpu cycles on just the interrupt code.  I've had much better luck with using
timers.  Also rememger you can get burned on the vertical blank since it can
be 50, 60, or 70hz. 

                               Wayne Knapp