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From: willmore@iastate.edu (David Willmore)
Subject: Re: IDE vs SCSI
Message-ID: <willmore.735435218@help.cc.iastate.edu>
Sender: news@news.iastate.edu (USENET News System)
Organization: Iowa State University, Ames IA
References: <1993Apr17.204247.6741@julian.uwo.ca>
Date: Wed, 21 Apr 1993 23:33:38 GMT
Lines: 61

wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
>wayne@amtower.spacecoast.org writes:

>IDE also uses DMA techniques.  I believe floppy controller also uses DMA,
>and most A/D boards also use DMA.  DMA is no big deal, and has nothing to
>do directly with SCSI.

No.  The simple $25 style IDE controller does not use DMA.  The CPU performs
the data transfer with a string move instruction.  This requires that the
CPU stop what it had been doing and transfer the data.  Only the smart drive
controllers, be they IDE or SCSI, can transfer via DMA.  These controllers
tend to cost the same wether they are IDE or SCSI.  To get the DMA benefits,
IDE must sacrifice it's price advantage *on the controller*.

>> For example, when rewinding or formatting a tape, the command is
>>issued to the controller and the bus is released to allow access to other
>>devices on the bus.  This greatly increases productivity or, at least, do
>>something else while backing up your hard drive :-).  Which happens to be
>>what I am doing while reading this group.

>You can thank your software for that.  If DOS had a few more brains, it
>could format floppies etc. while you were doing something else.  The
>hardware will support it, but DOS (at least) won't.  Again, this has   
>nothing to do with SCSI.

Floppies aren't on the IDE bus, your arguement makes no sense, this isn't
an IDE issue.  The floppys have their own processor on their controller
board which handles all of these commands.

The difference between IDE and SCSI is that all SCSI peripherials are
intelligent.  They each have their own controller.  They depend on the
CPU to do fewer things for them, i.e. IDE drives needing a processor
to transfer their data.

>>Its a long story, but I still use IDE on my 486 except for the CDROM which,
>>thanks to SCSI, I can move between both machines.  If, and when, SCSI is
>>better standardized and supported on the ibm-clone machines, I plan to
>>completely get rid of IDE.

>And if you stick with DOS you'll wonder why you can't multitask.

>Again I ask why can't a UNIX or OS/2 type OS do all the miraculous things
>with an IDE harddrive that it can with a (single) SCSI hard drive.

The dettach/callback mechanism alows the CPU to make requests of the 
devices on the SCSI bus and then dettach and go about its business.
Later, when the device is done, it issues a callback to say that the
data has arrived or the function has completed.  Most SCSI cards will
also DMA the data into memory without the interupting the CPU, therefore
allowing it to continue working uninterupted.  IDE supports no such concept
as dettach/callback.  

Can you see how this would be a win in any multitasking system?

--David Willmore

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David Willmore  | "Ever noticed how much they look like orchids? Lovely!" | 
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