Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!brunix!man
From: man@cs.brown.edu (Mark H. Nodine)
Newsgroups: comp.sys.mac.system
Subject: SCSI problems
Message-ID: <47848@brunix.UUCP>
Date: 21 Aug 90 18:25:57 GMT
Sender: news@brunix.UUCP
Reply-To: man@cs.brown.edu (Mark H. Nodine)
Organization: Brown University Department of Computer Science
Lines: 33

Ever since I upgraded my Mac 512Ke+2Meg memory to System 6.0.5 (I'm
using the XPRAM init, so that's not the problem), I've been having
problems with occasional system hangs.  I've traced the problem in
MacsBug to the code in the SCSI manager that implements the opcode
scInc (I don't know anything about SCSI).  The code around where the
hang occurs looks like

	MOVE.B	D7,$20(A4) <---------
	MOVEQ	#-$01,D1	    |
	MOVE.B	#$01,$20(A4)	    |
	BTST	#$06,$10(A3)	    |
	DBNE	D1		    |
	BNE.S	*+$A ------------   |
	MOVE.B	D7,$20(A4)	|   |
	MOVEQ	#$03,D0		|   |
	BRA.S	*+$2C --------	|   |
	MOVEQ	#$30,D0 <----+---   |
>>>	DBF	D0	     |      |
	BTST	#$5,$10(A3)  |      |
	BNE.S	*-$2C -------+-------
			     |
                             V

During execution, A3=$580000 and A4=$580003.  The ">>>" indicates
where the PC usually is when I break into the debugger during a hang.
The code seems to be called about every 2 seconds when there is
nothing in the SCSI unit (it's a cartridge drive) and only during disk
I/O when there is something there.

Any idea what could be the cause of this hang?  Do I have a faulty
SCSI port?  Or is there some incompatibility with System 6.0.5?

	--Mark