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From: sonny@charybdis.harris-atd.com (Bob Davis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Can I Low Level Format my MFM drive with 17 < SECTORS < 26 ?
Message-ID: <5855@trantor.harris-atd.com>
Date: 18 Mar 91 21:04:05 GMT
References: <5832@trantor.harris-atd.com> <1991Mar16.022315.28142@digi.lonestar.org>
Sender: news@trantor.harris-atd.com
Reply-To: sonny@trantor.harris-atd.com (Bob Davis)
Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL
Lines: 53

In article <1991Mar16.022315.28142@digi.lonestar.org> kgallagh@digi.lonestar.org (Kevin Gallagher) writes:
>In article <5832@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes:
>>
>>	I know that MFM normally uses 17 Sectors/Track and RLL uses 
>>26 Sectors/Track. But is there a fundamental reason I cannot low
>>level format my MFM drive to 20 Sectors/Track, say, in an attempt
>>to increase drive capacity? Does the MFM controller FORCE the
>>use of 17 S/T and the RLL controller 26 S/T or might they do whatever
>>they are told by the Low Level Formatter program?
>
>Without going into the technical details, the answer to your first question is
>yes.  Given the fixed RPM of your drive, the MFM encoding technique, and the
>speed at which the drive firmware writes to the disk, there is room for only
>17 sectors/track.  If you try to write an 18th sector on a track, you risk
>having part of the 18th sector overwrite part of the 1st sector.
>

	But I can take an MFM drive and RLL it to 26 sectors/track and there
is no problem with getting done with all 26 tracks in one revolution of
the platter.
Seems to me that the disk formatting routines in the BIOS must always know
how fast to write sector-definition info to the disk in order to complete
the specified number of sectors in exactly one revolution of the fixed
speed disk. Don't the BIOS routines do that?

	Some Low Level Formatting programs (such as Western Digital's WDFMT
program) allow the user to simply enter Cylinders, Heads, Sectors, and
all that other stuff and then it goes away and does its thing. Do such
programs do the right thing if you tell it to low level your MFM drive
using 18 sectors instead of 17?

	It simply occurs to me that MFM drives are designed to normally carry
a data density that corresponds to 17 sectors, but that some of the drives
are probably better than others and the platters might be of high enough
quality to carry 18 sectors, some 19, some even higher.

	My question is: Do the MFM controllers enforce the 17 sector choice
or might I use higher values? Do RLL controllers enforce the 26 sector choice?

	I realize that, even if possible, the choice of higher numbers would
stress the capability of the platters and that generally data would be at
a higher risk. Generally. In specific cases with better than average
drives, things might be OK...Just as some drives will RLL and some wont.

	I guess I will play with this on a drive soon and see what happens.


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