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Linux Tutorial - The Computer Itself - Floppy Drives
  Serial ATA ---- Tape Drives  


Floppy Drives

A customer once called in to tech support about a system that would not boot. For some unknown reason, the system crashed and would no longer boot from the hard disk. It got to a particular point in the boot process and hung. Even an old copy of the kernel hung in the same way.

Fortunately, the customer had an emergency boot floppy that enabled him to boot and gain access to the hard disk. The customer stuck the floppy in the drive and pressed the reset button. After a few seconds, there were the normal messages the system presented at boot up. For the moment, things looked fine. Suddenly the messages stopped and I could hear over the phone how the floppy drive was straining. It finally came up with the dreaded "floppy read error." Rather than giving up, I decided to try it again. Same thing.

At that point I started to get concerned. The hard disk booted, but the kernel hung. The floppy booted, but somewhere in the middle of loading the kernel, there was a bad spot on the floppy. This was not a good thing.

The floppy disk was brand new and the customer had tested it out immediately after he had made it. The most logical thing that caused this problem was putting the floppy too close to a magnetic field. Nope! That wasn't it, either. The customer had been told to keep this floppy in a safe place, and that's what the customer did.

What was that safe place? The customer had tacked it to the bulletin board next to the monitor, not through the hub or at one of the corners, but right through the floppy itself. The customer had been careful not to stick the pin through the media access hole because he was told never to touch the floppy media itself.

In this section, I'm going to talk about floppy disks, lovingly referred to as floppies. They come in different sizes and shapes, but all floppies serve the same basic functions. Interaction with floppies can be a cause of great heartache for the unprepared, so, Im going to talk about what they are like physically, how they are accessed, and what kinds of problems you can have with them.

Although they hold substantially less data than hard disks, floppies appear and behave very much like hard disks. Like hard disks, floppies are broken down into sectors, tracks, and even cylinders. Like hard disks, the number of tracks tells us how many tracks are on a given surface. Therefore, a floppy described as 40 tracks (such as a 360Kb floppy) actually contains 80 tracks, or 40 cylinders.

Other common characteristics are the header and trailer of each sector, which results in 571 bytes per sector, 512 of those being data. Floppy disks almost universally use MFM data encoding.

Linux floppy drivers support a wide range of floppies: from the ancient 48 tracks per inch/8 sectors per track, 5.25" floppies to the newest 135 tracks per inch/36 sector per track, 3.5" floppies that can hold almost 3Mb of data. More commonly however, the floppy devices found on systems today are somewhere in between these two types.

Because they are as old as PCs themselves, floppies have changed little except for their size and the amount of data that can be stored on them. As a result, very few problems are encountered with floppies. One most common problem is that customers are unsure which floppy device goes to which type of drive. Sometimes customers do know the difference and try to save money by forcing the floppy to format in a density higher than it was designed for.

The truth of the matter is, you can't format floppies higher than you're supposed to, that is, higher than the manufacturer specifies. To some extent, you might get away with punching holes in single-sided floppies to make them double-sided. However, forcing a floppy to a format at a higher density (if it works) isn't worth risking your data.

To understand why this is so, I need to talk about the concept of coercivity, that is, how much energy (how strong the magnetic field) must be used to make a proper recording on a disk. Older floppies had a lower coercivity and therefore required a weaker magnetic field to hold the signal; that is, less energy was required to "coerce" them into a particular pattern.

This seems somewhat contradictory, but look at it another way. As densities increased, the magnetic particles got closer together and started to interfere with each other. The result was to make the particles weaker magnetically. The weaker the particles are magnetically, the stronger a force was needed to "coerce" them into the proper patterns to hold data. Therefore, high-density disks have a higher coercivity.

As the capacity of drives increased, the tracks became narrower. The low density 5.25" floppies had 48 tracks per inch and could hold 360K of data. The high density 5.25" floppies have twice as many tracks per inch and can hold 1.2Mb (the added increase is also due to the fact they have 15 sectors per track instead of nine). Because there are more tracks in a given space, they are thinner. Problems arise if you use a disk formatted at 360K in a 1.2Mb drive. Because the 1.2 Mb drive writes the thinner tracks, not all of the track of the 360K floppy is overwritten. This may not be a problem in the 1.2Mb drive, but if you ever try to read that floppy in a 360K drive, the data will run together. That is, the larger head will read data from more than one track.

Formatting a 360K floppy as a 1.2Mb usually fails miserably because of the different number of tracks, so you usually cant get yourself into trouble. However, with 3.5" floppies, the story is a little different. For both the 720Kb and 1.44Mb floppies, there are 80 tracks per side. The difference is that the 1.44Mb floppies are designed to handle 18 sectors per track instead of just nine. As a result, formatting appears to go well. It is only later that you discover that the data is not written correctly.

The reason for this is that the magnetic media for the lower-density 720Kb floppies is less sensitive. By formatting it as 1.44Mb, you subject it to a stronger magnetic field than you should. After a while, this "overdose" causes the individual magnetic fields to begin to interfere with one another. Because high-density, 1.44Mb floppies cost a few cents apiece, it's not worth risking data by trying to force low-density to high-density to save money.

While I'm on the subject of money, note that buying unformatted floppies to save money is becoming less and less the smart thing to do. If you figure that formatting floppies takes at least two minutes apiece and the cost difference between a package of ten formatted floppies and ten unformatted is $2, then it would only make sense (or cents) to have someone format these if they were making only $6.00 an hour. Rarely does a company have someone whose sole job is to format floppies. That job usually falls on those people who use them and most of them get more than $6.00 an hour. Therefore, you may as well buy the formatted floppies.

(I actually did some consulting work for a company whose president insisted that they buy unformatted floppies. Because the only people who used the floppies were his programmers and system administrators, they earned more than $6.00 an hour. In one case, I calculated that turning a package of 10 unformatted floppies into formatted floppies worked out to costing twice as much for the unformatted as for the formatted ones. That didn't phase him a bit because the system administrators were on salary and were paid no matter what. By saving the few dollars by buying unformatted ones, his profit margin looked better, at least on paper.)

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.
  
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