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From: tas@mtuxo.UUCP (T.SKROBALA)
Newsgroups: comp.sys.amiga
Subject: Re: losing MIDI bytes
Message-ID: <755@mtuxo.UUCP>
Date: Mon, 21-Sep-87 02:13:31 EDT
Article-I.D.: mtuxo.755
Posted: Mon Sep 21 02:13:31 1987
Date-Received: Tue, 22-Sep-87 01:32:00 EDT
References: <8709110105.AA27649@cory.Berkeley.EDU> <27773@sun.uucp>
Organization: AT&T, Middletown NJ
Lines: 39
Summary: Chuck McManis and netnews come to the rescue


Thanks to all who responded to my question about losing MIDI system-exclusive
bytes.  I didn't have any luck with Forbid()/Permit(), and I wasn't keen
on busy waiting with all interrupts locked out, and I was indeed using
SetIntVector(), so I tried Chuck McManis'suggestion, and it worked
beautifully.  Now my Read Buffer Full handler has assembly code at the
end that does something like the following:

    for( i = 0 ; i < 50 ; i++ )
    {
	if( custom.intreq & INTF_RBF )
	    goto RBFHandler ; /* do it all again w.o. leaving the handler */
    }
    return ;

Just checking the bit once isn't good enough: apparently you can still
get screwed from time to time if the bit gets set right after you leave
the handler.  The loop does cause a few (~300) uSeconds of wasted time
if there is indeed nothing coming down the pike, but the gain in integrity
is worth it.  (I may be able to trim a few iterations off the loop and
still maintain integrity, but I know that there's a point where I start
losing again.)

By the way, when I was struggling with the problem, I saw loss of bytes
even when the MIDI data was only coming in bursts of 6 bytes, so I
imagine that the problem could arise even with just ordinary NOTE_ON/
NOTE_OFF types of stuff.  Anyway, Chuck's solution works, even with vd0:
running (not that I ever suspected that, but some people did), and I
suspect that serial.device will never work reliably unless it, too, has
the fix put in.

Now, can someone tell me just *what* was running and causing me to
lose those bytes before?  There isn't much that runs at priority
higher than the serial read interrupt, and I only lost about 1 byte
in 15000, i.e. about 1 every 5 seconds.  What high priority CPU-hog
thing happens on a more or less idle Amiga once every 5 seconds?
Am I guaranteed that Chuck's solution will *always* work?

Tom Skrobala  AT&T  mtuxo!tas