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? The Technology of ?
? Magnetic Disk Storage ?
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? by ?
? Steve Gibson ?
? GIBSON RESEARCH CORPORATION ?
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? Portions of this text originally appeared in Steve's ?
? InfoWorld Magazine TechTalk Column. ?
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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 **