Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!evax!utacfd!merch!cpe!adaptex!adaptx1!neese
From: neese@adaptx1.UUCP
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: How does an IDE drive work ?
Message-ID: <284500014@adaptx1>
Date: 27 Feb 91 05:28:28 GMT
References: <5222@male.EBay.Sun.COM>
Lines: 55
Nf-ID: #R:male.EBay.Sun.COM:5222:adaptx1:284500014:000:2677
Nf-From: adaptx1.UUCP!neese    Feb 26 10:06:00 1991


>/* ---------- "How does an IDE drive work ?" ---------- */
>Hi NetLanders,
>
>    I'm fascinated with the so-called IDE drives. I understand that the
>only thing a user has to do is to select a drive type closest to the
>capacity of the actual device during the CMOS setup, and the IDE drive will
>happily emulate that particular drive type. How is that done ?
>Specifically, I like to know the followings:
>
>	1. When does the drive figure out what to emulate ? At boot time ?

Some drive do it on the fly.  Other read the paramters at boot time and
others have switches to figure it out.  The best way to know about how
each drive does it is to get the OEM manual for that particular drive.

>	2. Who does the track/head/sector remapping ? The controller
>	   hardware, or the so-called expansion BIOS. Can somebody
>           describe what involves in the initialization phase and normal
>	   I/O operations ? Like reading the CMOS RAM, blah, blah...

The controller on the drive does the remapping.  As far as initialization
goes, it is best explained in the OEM manual for each drive.  It varies
from drive to drive.  But basic init is done by the system BIOS by the
code that did the init for standard disks.  IDE drives are *supposed* to
appear to the systems just like the standard ST506/MFM drives/controllers do.

>	3. Does that mean IDE drives depend heavily on the DOS operating
>	   environment in order to function ? If the drive fails to
>	   figure out what to emulate, does it act like a generic ST506
>	   disk controller ?

IDE drives do not depend on DOS for proper operation.  If the drive is
engineered correctly it will appear to be a standard ST506 MFM controller.

>	4. Now, the actual disk is a total black box as far as a user is
>	   concerned. What does a (high-capacity) disk look like,
>	   physically ? Does it use the standard ST506 data transfer rate ?
>	   Does it use a zone encoding technique, i.e. putting more sectors
>	   on outer tracks, to increase overall capacity?

As far as transfer rate goes,....it really doesn't matter how slow or how
fast the data transfer rate is.  As all data transfers take place via PIO
through the system bus, the system will take/send the data as fast as the
drive will send/receive the data.  The drive regulates the data transfer
rate, much like SCSI drives do the same thing.
The data rate to/from the drive will be at the rate of the encoding scheme.
The encoding scheme is completely transparent to the system.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  neese@adaptex
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