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From: tas@mtuxo.UUCP (T.SKROBALA)
Newsgroups: comp.sys.amiga
Subject: losing MIDI bytes
Message-ID: <689@mtuxo.UUCP>
Date: Mon, 7-Sep-87 13:16:25 EDT
Article-I.D.: mtuxo.689
Posted: Mon Sep  7 13:16:25 1987
Date-Received: Tue, 8-Sep-87 03:10:42 EDT
Distribution: comp
Organization: AT&T, Middletown NJ
Lines: 53


I have an application which uses serial.device to read bursts of ~400 MIDI
"system exclusive" bytes, and I've had problems with occasional lost
bytes, even with SERF_RAD_BOOGIE set.  So I'm looking into the
possibility of writing my own serial device driver.  But I *still* seem
to lose bytes, even with a darn-near trivial interrupt handler.  Right
now I have code that looks something like this:

	long byteCount ;

	main()
	{
	    byteCount = 0 ;
	    initMIDI( &byteCount ) ;
	    getchar() ;	/* wait until RETURN is hit */
	    printf( "%ld\n", byteCount ) ;
	    cleanupMIDI() ;
	}

and a serial "read buffer full" interrupt handler that is:

	_RBFHandler:
	    move.w	SERDATR(a0),d1		; Copy input byte/word to d0.
	    move.w	#INTF_RBF,INTREQ(a0)	; Clear interrupt.
	    add.l	#1,(a1)			; Inc count pointed to by a1.
	    rts

I have the 1.2 operating system, 2 floppy drives, and 2 Mbytes of Fast RAM.
In my tests, I do not have any background processes or other devices running
unless otherwise noted.  I have set the SERPER register to both 113 and 114
for 31250 bits per second.

The above code, with no other processes running, will generally not lose
bytes.  But if I add a single process (popCLI) in the background, or if
I add about 15 microseconds worth of data collection code to the
interrupt handler, I start seeing data loss of 1 or more bytes per 20K
(the 20K always in bursts of 400 bytes, the bursts at close to the full
MIDI bandwidth of 3125 bytes per second).  When I lose a byte, I usually
see that the OVRUN bit has been set in the SERDATR register.

Is there a bug in my simple handler?  Or is there some system overhead
that is occasionally locking out serial interrupts for a significantly
large percentage of the 300+ microseconds between MIDI data bytes?  If
the latter, is there a workaround?

Tom Skrobala  AT&T  mtuxo!tas  201-957-5446

P.S. I've had similar problems with a timer interrupt handler.  I start
losing 1 percent and more of my interrupts when I get above 2000
interrupts per second or so.  The timer is programmed to fire
repeatedly.  That interrupt handler simply decrements a counter and
sends a signal to the application process if the counter has gone down
to 0.