Path: utzoo!mnetor!uunet!wyse!weitek!pyramid!prls!philabs!micomvax!ray
From: ray@micomvax.UUCP (Ray Dunn)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Disk holes and other strange things
Message-ID: <930@micomvax.UUCP>
Date: 8 Mar 88 21:32:46 GMT
References: <3037@cup.portal.com> <3186@cup.portal.com> <3255@cup.portal.com> <2660@gryphon.CTS.COM> <1414@puff.cs.wisc.edu>
Reply-To: ray@micomvax.UUCP (Ray Dunn)
Followup-To: alt.flame
Organization: Philips Electronics Ltd. (TDS - Montreal) St. Laurent P.Q., Canada
Lines: 70

[Please excuse late news processing]

To protect the rude and the ignorant, I shall leave off names...

Mr A:
 >The index hole on the floppy disk is used only for formatting purposes so all
 >tracks remain in sync.  When the disk is read, the hole is not used.  I'm not
 >sure about writes.

Mr B:
 > Don't do a term paper on index holes without some more research.
 > 
 > The index hole to which you refer, as opposed to index holes that
 > demarcate sectors on a hard sectored diskette, allows the controller
 > to determine:
 >    1)  That the diskette is moving.
 >    2)  That the diskette has made a complete revolution (the hole went by
 >        twice.)
 > In addition, it provides a convenient start-of-track reference marker
 > when formatting the diskette.  Diskette tracks are not in sync with
 > anything, by the way.

Mr A:
 >First of all, try this out on your PC.  Cover the index hole of any formatted
 >disk.  If it can boot, boot it.  Surprise!  Your PC is reading the disk quite
 >well without the aid of the hole.  Try DIR, surprise again!  It also works
 >and it didn't even have to use the index hole.  Now try to create a file.
 >  Hey!
 >That works too, and it didn't even complain with an error like INDEX HOLE
 >COVERED STUPID, I CAN'T READ YOUR DISK.
 >The ONLY time DOS uses that index hole
 >is when it formats.  At least we agree on that.
 >

If you guys would stop shouting at each other, you Mr A would learn
something, and you Mr B would be able to communicate your ideas better and
receive some respect from your peers.

The **facts** are quite simple, and are as stated by Mr B:

On an IBM type disk drive and controller:

On a soft sectored diskette, the index hole is used as a start of track
marker to indicate to the FORMAT process when it should start writing the
soft sectoring information.  Within the track, the sectors can be ordered in
any way you like.  I know of no disk controller which will abort the track
format command if the index hole is seen again during the track format.

During disk READING, the required sector is searched for by reading the
sector headers written during the formatting, the controller does not need
to wait for the index hole before it starts its search.  If the sector is
found *BY THE TIME THE INDEX HOLE IS SEEN FOR THE SECOND TIME* then the data
is read.  Otherwise an error is flagged, as the addressed sector cannot be
found.

A similar process takes place when the controller is searching for the
sector header of the sector to be written.

If you cover the index hole and then attempt to read an illegal sector, or
one whose header has been corrupted, the search will continue ad pensionem!

If I remember the distant past, the 1791 controller also used the index hole
to determine whether a disk was loaded & spinning.

Thus Mr A, it is only when things start to go *wrong* that the index hole
has a bearing on reading or writing.  But it is *IS* required!!!  Stop
shouting - start thinking!!!  If you Mr B had not opened your response with
an insult, Mr A might have listened to you!!

Ray Dunn.  ...{philabs, mnetor, musocs}!micomvax!ray