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From: jcburt@ipsun.larc.nasa.gov (John Burton)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: What is difference between Unformatted and Formatted HD capacity?
Message-ID: <1991Apr5.132126.27868@news.larc.nasa.gov>
Date: 5 Apr 91 13:21:26 GMT
References: <1991Apr4.203413.7550@news.larc.nasa.gov> <5994@trantor.harris-atd.com>
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In article <5994@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes:
[...]
>	I have seen some drives having voice coil head positioning
>mechanisms that have an odd number of heads. But certainly all drives
>have an even number of platter surfaces. Is the missing head's
>platter and its bytes counted as part of the "Unformatted Capacity"?
>
>	Why is there a missing head on such drives? Is that head
>*really* there and it is just that the platter surface is used for 
>some overhead function? If so, what?
>
>Thanks.
>_____________________________________________________________________________
>Bob Davis, UofALA alum \\ INTERNET: sonny@trantor.harris-atd.com  |  _   _  |
>Harris Corporation, ESS \\    UUCP: ...!uunet!x102a!trantor!sonny |_| |_| | |
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Sorry I can't help you with the formatted vs. unformatted capacity question...

Concerning voice coil drives...you're right, there *is* an even number of
surfaces and there *is* an even number of physical heads. With a voice coil
drive, the "missing" head (and therefore surface) is used for track positioning
information - during formatting, special info is written to each track on the
"extra" surface basically stating "I am track xxx". During normal r/w operations
the actuator moves the heads until the extra head reads the proper "I am track xxx"
information, at which point the regular r/w heads are positioned correctly.
This eliminates (or at least greatly reduces) the problem of thermal expansion, since
all the platters have the same expansion characteristics, and the same environment. 
With stepper motors, the proper track is found by moving the head xxx number of steps
from its starting position. This DOES NOT take into account the thermal expansion
of the disk media since it is an absolute positioning system (position in space)
vs. a relative positioning system (position relative to the disk). I know it sounds 
like a lot of extra bother, but the voice coil normally has faster seek times
(most of your < 25msec seek time drives are voice coil - I don't know of any
stepper motor drives with that kind of seek times, but I'm sure there are some) 
and gets you the freebie of Autoparking heads...when the power goes off, the heads
automatically return to their park position, no extra circuits required...

hope this helps some...

John
+--------------------------------------------------------------------+
| John Burton                                                        |
| G & A Technical Software                                           |
| jcburt@gatsibm.larc.nasa.gov                                       |
| jcburt@cs.wm.edu                                                   |
|                                                                    |
| Disclaimer: Hey, what can I say...These are *my* views, not those  |
|             of anyone else, be they employer, school, or government|
+--------------------------------------------------------------------+