Newsgroups: comp.periphs.scsi
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!nuchat!steve
From: steve@nuchat.sccsi.com (Steve Nuchia)
Subject: Re: SCSI 2 Command Queing
Message-ID: <1991Jun15.160002.24952@nuchat.sccsi.com>
Organization: South Coast Computing Services, Inc.  Houston
References: <31293@hydra.gatech.EDU>
Distribution: comp.periphs.scsi
Date: Sat, 15 Jun 91 16:00:02 GMT

In <31293@hydra.gatech.EDU> jt34@prism.gatech.EDU (THOMPSON,JOHN C) writes:
>SCSI 2 supports tagged command queing which I think means that the host may
>send multiple command requests to the same device. Do most multitasking OSs
>allow this? Say for example in an UNIX environment that task 1 made a request

I have no idea how to answer for "most" OSs.  For Unix the answer is
a qualified "of course".  All you need is a driver that knows that
the hardware can handle it.  The normal disk drivers (block device drivers)
all have software queueing of requests.  The "elevator" sort is performed
on the contents of this queue, and one entry is sent out whenever the
controller is ready.  The driver would have to be restructured to do
it, but it certainly can be done.  Of course, no such drivers exist in
the PC Unix world today.


-- 
Steve Nuchia	      South Coast Computing Services      (713) 964-2462
	"Innocence is a splendid thing, only it has the misfortune
	 not to keep very well and to be easily misled."
	    --- Immanuel Kant,  Groundwork of the Metaphysic of Morals