The Technology of Magnetic Disk Storage

Published in

· 5 years ago

 
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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 -