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From: hulsebos@philmds.UUCP (Rob Hulsebos)
Newsgroups: comp.unix.wizards,comp.unix.questions,comp.periphs
Subject: Formatting disk theory
Keywords: format, disks, formatting program
Message-ID: <619@philmds.UUCP>
Date: 24 Aug 88 14:28:15 GMT
Organization: Philips I&E DTS Eindhoven
Lines: 42

I have the experience that most people who need to format a disk do not
know anything about how this should be properly done. Most of the programs
that I saw just format the disk with a 'standard' pattern of 0xE5.

As I once read somewhere, this is the pattern for a single-density disk
(which uses the FM-coding mechanism). Single density is, however, something
used in the past: the very first micro that I worked on had single-density
floppies holding 120K (!) each.

But now, most disks (at least the disks that I use) have the MFM-coding
mechanism, also known as "double density". But the 0xE5 pattern is still
used, which is not correct: double-density disks must be formatted with
the 0xD6B6 pattern.

A different pattern is necessary because data is stored differently. The
0xD6B6 is some kind of worst-case pattern for MFM, so flaws on the disks
can usually be found.

QUESTION 1: Who knows more about this subject ? Any papers ?

I also heard that either for VAX/VMS or Ultrix or BSD a special diskformat
program exists which runs 40 or more passes over the disk, in an attempt
to find all bad spots on it and also any marginal sectors. It seems that
if you write the same data repeatedly to the same sector, and the sector
is not 100% OK, after a certain time the sector will appear to be bad.
Probably other algorithms are also used in this program to find more
errors.

QUESTION 2: Is this program public domain ? Has there ever been a 
            proper description of the algorithm used ?

Of course, with the current SCSI-disks it is fairly easy to work around
bad spots on a disk. But what I like about the 40-pass program is that it
can also detect marginal sectors, which are now still OK. And it is always
better to prevent those problems than to wait for them to appear.

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R.A. Hulsebos                                       ...!mcvax!philmds!hulsebos
Philips I&E Automation Modules                            phone: +31-40-785723
Building TQ-III-1, room 11
Eindhoven, The Netherlands                                # cc -O disclaimer.c
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