Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!ur-tut!sunybcs!boulder!hao!oddjob!uwvax!puff!ttang
From: ttang@puff.cs.wisc.edu (Theodore Tang)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Disk holes and other strange things (was Re: F-15 Strike Eag)
Summary: whoa there, try again
Message-ID: <1414@puff.cs.wisc.edu>
Date: 21 Feb 88 23:39:42 GMT
References: <3037@cup.portal.com> <3186@cup.portal.com> <3255@cup.portal.com> <2660@gryphon.CTS.COM>
Organization: U of Wisconsin CS Dept
Lines: 54

In article <2660@gryphon.CTS.COM>, greg@gryphon.CTS.COM (Greg Laskin) writes:
> In article <1412@puff.cs.wisc.edu> ttang@puff.cs.wisc.edu (Theodore Tang) writes:
> >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.
> >
> 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.

If FORMAT uses the index hole as a reference to the start of track, I do
believe that is syncronizing the tracks so they all start right after the
appearance of the index hole.
 
> If, for example, you were looking for sector 4 and two index holes
> went by, it would be a good bet that you weren't going to find sector 4.
> Thanks to our friend, the hole, sectors don't have to be recorded
> sequentially on the diskette.

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.

Hole or no hole, sectors are scewed (scrambled) so access time is faster to
find sectors.

> If you see two index holes while formatting a track, you've written too
> much formatting on the track, although many controllers don't recognize
> this one.
> 
> -- 
> Greg Laskin           

Theodore Tang
University of Wisconsin at Madison

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