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From: guyd@austin.ibm.com (Guy Dawson)
Subject: Re: IDE vs SCSI, DMA and detach
Originator: guyd@pal500.austin.ibm.com
Sender: news@austin.ibm.com (News id)
Message-ID: <C5uK1y.1Ao8@austin.ibm.com>
Date: Wed, 21 Apr 1993 18:44:21 GMT
References:  <1993Apr19.034517.12820@julian.uwo.ca>
Organization: IBM Austin
Lines: 60


In article <1993Apr19.034517.12820@julian.uwo.ca>, wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
> In article <RICHK.93Apr15075248@gozer.grebyn.com> richk@grebyn.com (Richard Krehbiel) writes:
> >>     Can anyone explain in fairly simple terms why, if I get OS/2, I might 
> >>   need an SCSI controler rather than an IDE.  Will performance suffer that
> >>   much?  For a 200MB or so drive?  If I don't have a tape drive or CD-ROM?
> >>   Any help would be appreciated.
> 
> >So, when you've got multi-tasking, you want to increase performance by
> >increasing the amount of overlapping you do.
> >
> >One way is with DMA or bus mastering.  Either of these make it
> >possible for I/O devices to move their data into and out of memory
> >without interrupting the CPU.  The alternative is for the CPU to move
> >the data.  There are several SCSI interface cards that allow DMA and
> >bus mastering.
>  ^^^^^^^^^^^^
> How do you do bus-mastering on the ISA bus?
> 
> >IDE, however, is defined by the standard AT interface
> >created for the IBM PC AT, which requires the CPU to move all the data
> >bytes, with no DMA.
> 
> If we're talking ISA (AT) bus here, then you can only have 1 DMA channel
> active at any one time, presumably transferring data from a single device.
> So even though you can have at least 7 devices on a SCSI bus, explain how
> all 7 of those devices can to DMA transfers through a single SCSI card
> to the ISA-AT bus at the same time.

Think!

It's the SCSI card doing the DMA transfers NOT the disks...

The SCSI card can do DMA transfers containing data from any of the SCSI devices
it is attached when it wants to.

An important feature of SCSI is the ability to detach a device. This frees the
SCSI bus for other devices. This is typically used in a multi-tasking OS to
start transfers on several devices. While each device is seeking the data the
bus is free for other commands and data transfers. When the devices are
ready to transfer the data they can aquire the bus and send the data.

On an IDE bus when you start a transfer the bus is busy until the disk has seeked
the data and transfered it. This is typically a 10-20ms second lock out for other
processes wanting the bus irrespective of transfer time.

> 
> Also, I'm still trying to track down a copy of IBM's AT reference book,
> but from their PC technical manual (page 2-93):
> 
> "The (FDD) adapter is buffered on the I.O bus and uses the System Board
> direct memory access (DMA) for record data transfers."
> I expect to see something similar for the PC-AT HDD adapter.  
> So the lowly low-density original PC FDD card used DMA and the PC-AT
> HDD controller doesn't!?!?  That makes real sense.
-- 
-- -----------------------------------------------------------------------------
Guy Dawson - Hoskyns Group Plc.
        guyd@hoskyns.co.uk  Tel Hoskyns UK     -  71 251 2128
        guyd@austin.ibm.com Tel IBM Austin USA - 512 838 3377
