Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!julius.cs.uiuc.edu!apple!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.periphs.scsi
Subject: Re: Command Queueing in SCSI-2
Message-ID: <35475@cup.portal.com>
Date: 1 Nov 90 13:55:16 GMT
References: <4297@alliant.Alliant.COM>
Organization: The Portal System (TM)
Lines: 32

Yes.  When you select the target with ATN, you can follow the
identify with a queue tag message.  This is a two byte message.
The second byte is an identifier that identifies the request.
The target is allowed to queue such requests.

When the target reselects, it will send this tag back so you
can tell which I/O is continuing.

The first byte of the message tells how the request is to
be placed in the queue.  There are three possible values:

	HEAD OF QUEUE

		The request goes to the head of the queue

	SIMPLE

		The request goes in the queue.  Requests can
		be reordered by target subject to the conditions
		below.

	ORDERED

		The request goes in the queue.  All SIMPLE or
		ORDERED requests that are already in the queue
		will be completed before this request.

		All subsequent SIMPLE or ORDERED requests that
		arrive after this one will be completed after
		it.

						Tim Smith