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The Technology of Magnetic Disk Storage

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        К                                                              К
        К                       The Technology of                      К
        К                     Magnetic Disk Storage                    К
        К                                                              К
        К                              by                              К
        К                         Steve Gibson                         К
        К                  GIBSON RESEARCH CORPORATION                 К
        К                                                              К
        К                                                              К
        К     Portions of this text originally appeared in Steve's     К
        К               InfoWorld Magazine TechTalk Column.            К
        К                                                              К
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        The technologies used to store and retrieve data to floppy and
        hard disks is intriguing, intuitive, and surprisingly simple.
        This article examines the technology of disk data storage.  Soon 
        you'll know exactly how and why RLL hard disk controllers are 
        able to pack 50 percent more data onto your trusty old reliable 
        hard disk ... and why they may NOT be giving you something for 
        nothing! 

        It all begins with two intimately related phenomena: magnetism
        and electricity.  Just as a flow of electric current has a
        direction which can be called positive or negative,  magnetism
        has a direction known as north and south poles.  Recalling high
        school physics, you'll remember that an electric current flowing
        through a coil of wire creates a magnetic field, and conversely,
        a change in a magnetic field near to a coil INDUCES a flow
        of electric current.  If we add to this a metal's ability to
        "remember" a magnetic field's direction by becoming magnetized,
        we have everything we need for storing and retrieving
        information.

        The read/write head in a slow-spinning floppy disk stays in
        physical contact with the disk medium at all times while the
        faster rotation rate of a hard disk causes its head to
        aerodynamically FLY over the disk's surface when the drive is up
        to operating speed.  Since a drive's read/write head and disk
        "communicate" using magnetic fields, and since magnetic fields
        travel through the air readily, actual physical contact between
        the head and disk is not necessary.  The disk drive's head and
        disk only need to be close enough to magnetically "couple" and
        influence each other as a result.

        A disk's read/write head is a specially designed coil of wire
        wrapped around a metal armature.  This armature has a very tiny
        GAP across which the magnetic field generated by the coil JUMPS.
        The gap serves to concentrate the jumping magnetic field into a
        tiny spot on the disk.  As the field jumps the gap, a bit of
        magnetic field protrudes from the head and passes through the
        nearby disk or diskette.  When a read/write head wears out it's
        because this gap has widened, becoming too large, and thus
        has lowered the resolution of the head.

        Writing data onto a disk takes advantage of magnetization.  An
        electric current is applied to the coil in the disk head.  This
        produces a magnetic field which jumps across the gap of the head
        and protrudes into the disk surface.  Since disks are composed
        of a metallic oxide, tiny spots of the disk become magnetized
        and thus "remember" the magnetic field which was imposed.

        Reading data is essentially the writing process in reverse.  The
        tiny magnetic spots on the disk create their own tiny protruding
        magnetic fields.  As the disk rotates, the disk head passes over
        these tiny protruding fields.  When these fields fall across the
        gap in the read/write head a small electric current is induced
        in the head's wire coil.  A sensitive READ AMPLIFIER boosts this
        signal up to useable strength for interpretation as the data
        stored on the disk.

        The question now is:  How do we ERASE the little magnetized
        blips on our disk to allow us to CHANGE the data recorded there?
        So far all we could do would be to magnetize the entire track,
        which wouldn't help us either!  The answer lies in the fact that
        it is a CHANGE in the magnetic field which induces a recoverable
        flow of current.  (After all, if a fixed magnetic field were
        able to produce a steady current flow in a surrounding wire coil,
        we'd have the equivalent of perpetual motion ... or perpetual
        power!)  Remember that magnetic fields are like electric current
        in that they're either present or not, and they have a distinct
        direction, a north or south polarity!

        When we're WRITING data onto a disk we don't turn the current on
        and off, we keep current flowing through our read/write head at
        all times.  When we wish to write a "ONE" bit, we simply REVERSE
        the POLARITY of the head's current.  This reverses the recorded
        magnetic field from north to south or south to north.  We don't
        care which way the field changes since ANY reversal represents a
        "one" bit and no reversal represents a "zero."

        Since we have an electric current of one polarity or the other
        flowing through the head at all times, the constant magnetic
        field produced "plows over" any old "blips" or polarity
        reversals which might have been present before.  This
        effectively leaves "zeros" in our wake except where we
        deliberately reverse the polarity to leave a "one" bit instead.

        So what are the various factors which determine the upper limits
        on the number of "ones" and "zeros" a disk can hold and the finer
        points of data storage encoding and density?

        We've seen that "one" bits are written onto floppy and hard disks
        by reversing the polarity of the current passing through the
        drive's read/write head.  "Zero" bits are written simply by not
        reversing that polarity.  These polarity reversals cause a
        DIRECTION reverse of the magnetic field "flux" imposed by the
        read/write head upon the disk.  The data storing "memory" effect
        of a disk comes from the metallic nature of the disk's oxide
        coating which becomes magnetized with these patterns of "flux
        reversals."  During data read-back these flux reversal patterns
        induce a weak current pulse in the read/write head which is
        amplified by the read amplifier and used to recover the stored
        data.

        This data recording scheme leaves us with a major problem:
        Reading back "ones" is simple since a pulse is received from the
        read/write head for every flux reversal encountered, but "zeros"
        are another matter entirely!  Since "zeros" are "written" by
        writing nothing, we can't be certain exactly how many "zeros" were
        written between the "ones!"

        In theory we could measure the TIME between successive "one"
        pulses and infer how long the RUN of "zeros" must have been, but
        this is
        too uncertain when we have unlimited run lengths.  The first
        single-density floppy disk controllers used a simple data
        encoding scheme to solve this problem.

        A "zero" data bit was actually written as a one-zero pulse pattern
        (a pulse and a pause) on the disk and a "one" was written as a
        "one-one" pattern (two pulses).  In this coding scheme the first
        pulse, known as the clock-bit, was always present, and the second
        pulse, known as the data-bit, was the actual data to be written.

        Writing five "ones" in this scheme would produce a pulse pattern
        of 1111111111 on the disk while writing five "zeros" produces
        1010101010.  Since the frequency of pulses for "one" data bits is
        twice that for "zeros" this scheme was known as FREQUENCY
        MODULATION or "FM" encoding.  In FM the minimum RUN LENGTH of no
        flux reversal pulses is zero since there might be no pauses at all
        between pulses and the maximum pause run length is "one" since the
        interposed "clock bits" guarantee at least a one pulse every
        other time.  A notational shorthand for this scheme would be
        "0,1 RLL."  (getting the picture?)

        This simple encoding scheme worked wonderfully.  Everyone was
        happy, felt good, and smiled a lot.  However after a while,
        people began to want more.  The problem with the FM modulation
        scheme is that it was inefficient.  It used up lots of pulses
        since a "one" data bit used two pulses and a "zero" used one.  It
        required an average of one and a half pulses per data bit.

        One way of increasing the density would have been to put the
        pulses closer together, but they were ALREADY as close together
        as they could be!  So a bright engineer came up with a clever
        solution:  If we promised to always have a least ONE pause
        between pulses, we could put the pulse patterns out twice as
        fast!  Then two twice-as-fast pulses separated by one pause
        would be no closer than two pulses right next to each other had
        been before!

        This coding scheme is called MFM for MODIFIED Frequency
        Modulation.  A "one" bit's pulse pattern is 01, and a 0 is x0
        where
        x was a pause if there had just been a pulse and a pulse if
        there had just been a pause.  Twiddling around with this on a
        napkin you'll see that this always forces at least 1 no-pulse
        pause between pulses and never allows more than 3 pauses between
        pulses.  Since this MFM coding scheme doubles the data rate over
        FM, it is called double-density and could also be called 1,3 RLL
        since the pause run lengths are limited between 1 and 3. All
        standard floppy and hard disk today use this MFM or 1,3 RLL
        encoding.

        Then when we began wanting even more density the way was clear.
        2,7 RLL, known today simply as "RLL,", cranks the data bit rate,
        and therefore the density, up 50 percent higher by guaranteeing at
        least 2 (very short) pause intervals between successive pulses
        and limiting the pause run length to 7.

        Another way of looking at this will show you what's REALLY
        HAPPENING here:  We've been cranking the data rate and data
        density upwards while promising not to place successive pulses
        closer together.  We've been squeezing more INFORMATION out of
        the same overall NUMBER of pulses by using their EXACT POSITION
        IN TIME to carry the information.

        The EXACT TIMING PLACEMENT of the pulses is used to convey more
        information than the pulses alone could!  This is why many hard
        disk drives which work wonderfully for MFM encoded data WILL NOT
        FUNCTION RELIABLY with the new 2,7 RLL controllers.  These RLL
        controllers demand far more accuracy from the drive's magnetic
        systems than they were ever designed to deliver.


        So what about RLL controllers and MFM drives?

        The thought of exchanging an existing MFM hard disk controller
        for an RLL controller is quite captivating.  By placing 25 or 26
        sectors on a track, RLL controlllers deliver a 50 percent storage
        gain over standard MFM controllers with their 17 sectors.  Ten
        megabyte drives hold 15 megs. and 20s become 30s.

        Aside from sheer storage space there is another unexpected
        advantage to RLL.  Imagine that your disk initially held 20
        megabytes with MFM encoding.  Converting to RLL encoding now
        yields 30 meg.  Notice that the original 20 megs have been
        squeezed down.  Now they occupy only 2/3 of the disk.  This means
        that your drive's read/write head only moves 2/3 as far as before
        to reach the same data!  In effect you've SUBSTANTIALLY REDUCED
        the average seek time of your drive ... for free!

        This is something most people completely fail to take into
        account with hard disk drives.  The time to move the read/write
        head from track to track is NOT the whole story.  It's critical
        to consider how much data that track-to-track move COVERS.  A
        drive with more storage platters (and heads) or more sectors per
        track has a greater "cylinder density."  RLL automatically
        increases a drive's cylinder density.

        RLL also affects the optimal interleaving factor for a drive!
        Remember that MFM and RLL utilize essentially the same number of
        flux reversals per inch.  However RLL utilizes infinitesimal
        timing placements of the pulses to convey more information.
        This means that the actual recovered data rate is 50 percent
        higher.

        Data flows from an RLL encoded drive at 7.5 million bits per
        second, as opposed to 5 million bits per second for MFM.
        Unfortunately PC and XT busses are already pushed to the limit
        by the optimal sector interleave of existing MFM controllers.
        Therefore RLL controllers require a LOOSER optimal interleave
        than MFM controllers.  This does not mean that RLL controllers
        operate slower, quite the opposite is true.  Since the PC bus is
        not able to take data any faster, and since there are now 25 or
        26 sectors per track, it's completely reasonable to require more
        revolutions of the disk to read or write 50 percent more data.

        It is much more critical to optimize the sector interleave for
        RLL encoding than for MFM.  The latest RLL controller from WD is
        the nicest I've seen, however using their default interleave of
        3 on a standard 4.77 Mhz PC or XT requires 28 revolutions to
        read an entire track!  Setting the interleave to 4 allows the
        same data to be read in JUST 4 REVS!  A 700 percent performance
        boost, free!

        Now for the bad news:  Many people have had trouble with RLL
        controllers.  This is typically caused by the hope that an RLL
        controller's magic will function with any MFM-compatible drive.
        We've seen why this may not be so.  It also appears that hard disk
        drive manufacturers, eager to cash in on the RLL craze,
        have merely been labeling the best of their MFM drives as RLL
        capable, rather than re-engineering their drives for RLL
        operation.  RLL is still so new that adequate drive testing
        equipment is in very short supply.

        Make no mistake, RLL encoding is the future.  These initial
        startup growing pains will fade and RLL technology will become
        the new standard.

                                   - The End -


                     Copyright (c) 1989 by Steven M. Gibson
                             Laguna Hills, CA 92653
                            **ALL RIGHTS RESERVED **