Floppy Disk Drives

In the hard disk drive chapter, I refer to the hard disk as the "data center" of the PC, and in fact it is, but there was a time when the floppy disk actually held this honorific. In fact, the first PCs didn't have hard disks; all of their data storage was done on floppies. There was a time when floppy disk drives were high technology and cost serious money. I remember paying $700 for a floppy disk for my Apple II PC about 15 years ago... and being very excited about not having to use a cassette deck any more to load and save programs. Talk about high technology.

The invention of hard disks relegated floppy disks to the secondary roles of data transfer and software installation. The invention of the CD-ROM and the Internet, combined with the increasingly large size of software files, is threatening even these secondary roles. The floppy disk still persists, basically unchanged for over a decade, in large part because of its universality; the 3.5 inch 1.44 MB floppy is present on virtually every PC made in the last 10 years, which makes it still a useful tool. The floppy disk's current role is in these area:

• Data Transfer: The floppy disk is still the most universal means of transferring files from one PC to another. With the use of compression utilities, even moderate-sized files can be shoehorned onto a floppy disk, and anyone can send anyone a disk and feel quite confident that the PC at the other end will be able to read it. The PC 3.5" floppy is such a standard, in fact, that many Apple and even UNIX machines can read them, making these disks useful for cross-platform transfer.
• Small File Storage and Backup: The floppy disk is still used for storing and backing up small amounts of data, probably more than you realize.
• Software Installation and Driver Updates: Many new pieces of hardware still use floppies for distributing driver software and the like, and some software still uses floppies (although this is becoming less common as software grows massive and CD-ROM drives become more universal.)

While floppy drives still have a useful role in the modern PC, there is no denying their reduced importance. Very little attention is paid to floppy "performance" any more, and even choosing makes or models involves a small fraction of the amount of care and attention required for selecting other components. In essence, the floppy drive today is a commodity item! For this reason, I examine the floppy drive in this chapter but do not go into a great level of detail. In addition, since many aspects of floppy disk construction and logical operation are similar to those of hard disks, and since I did describe hard disks in a great level of detail, I make frequent references back to relevant sections in the chapter on hard disks.

Floppy Disk Drive Construction and Operation

While floppy disk drives vary in terms of size and the format of data that they hold, they are all internally quite similar. In terms of construction and operation, floppy drives are similar to hard disk drives, only simpler. Of course, unlike hard disks, floppy disk drives use removable floppy media instead of integrated storage platters. This section takes a look at the basic components and physical operation of a floppy disk drive.

The read/write heads on the floppy disk are used to convert binary data to electromagnetic pulses, when writing to the disk, or the reverse, when reading. This is similar to what the heads on a hard disk do.

There are several important differences between floppy disk and hard disk read/write heads. One is that floppy disk heads are larger and much less precise than hard disk heads, because the track density of a floppy disk is much lower than that of a hard disk. The tracks are laid down with much less precision; in general, the technology is more "primitive". Hard disks have a track density of thousands of tracks per inch, while floppy disks have a track density of 135 tracks per inch or less.

In terms of technology, floppy disks still use the old ferrite style of head that was used on the oldest hard disks. In essence, this head is an iron core with wire wrapped around it to form a controllable electromagnet . The floppy drive, however, is a contact recording technology. This means that the heads directly contact the disk media, instead of using floating heads that skim over the surface the way hard disks do. Using direct contact results in more reliable data transfer with this more simplistic technology; it is impossible to maintain a consistent floating head gap at any rate when you are using flexible media like floppies.

Since floppy disks spin at a much slower speed than hard disks--typically 300 to 360 RPM instead of the 3600 RPM or more of hard disks--they are able to contact the media without causing wearout of the media's magnetic material. Over time, however, some wear does occur, and magnetic oxide and dirt builds up on the heads, which is why floppy disk heads must be periodically cleaned. Contact recording also makes the floppy disk system more sensitive to dirt-induced errors, cause by the media getting scratched or pitted. For this reason, floppy disks are much less reliable, overall, than hard disks.

The floppy disk also uses a special design that incorporates two erase heads in addition to the read/write head. These are called tunnel-erase heads. They are positioned behind and to each side of the read/write head. Their function is to erase any stray magnetic information that the read/write head might have recorded outside the defined track it is writing. They are necessary to keep each track on the floppy well-defined and separate from the others. Otherwise interference might result between the tracks.

An analogy to demonstrate how these work would be as follows. Imagine a man laying a gravel walkway on a narrow dirt path, by walking a wheelbarrow along it that has a slit in the bottom cut to the width of the path, through which the gravel falls. Most of the gravel will fall on the dirt path but some will fall on the grass to either side of the path. The tunnel-erase heads would be like two helpers that follow behind the man with the wheelbarrow and "pick up" any gravel that has fallen on the grass instead of the path. (Of course gravel can't cause "interference" the way magnetic patterns can.)

All modern--and even not-so-modern--floppy disks are double-sided. Very, very old floppy disks originally were single-sided only. Since the disks are double-sided, there are two heads, one per side, on the drive. The heads contact the media on each side by basically squeezing the media between them when the disk is inserted. The heads for different drives vary slightly based on the drive format and density.

The head actuator is the device that physically positions the read/write heads over the correct track on the surface of the disk. Floppy disks generally contain 80 tracks per side. The actuator is driven by a stepper motor. As the stepper motor turns it moves through various stop positions, and in doing so, moves the heads in and out one or more position. Each one of these positions defines a track on the surface of the disk.

Stepper motors were originally used for the actuators for hard disks as well, but were replaced by voice coils due to problems with reliability and speed. Since the stepper motor uses pre-defined track placements, thermal expansion in hard disks can cause errors in older hard disks that use stepper motor actuators, when the disk platters expand and move the tracks to a place different than where the heads are expecting them. This is not an issue for floppy disks because of their much lower track density, plus the fact that thermal expansion isn't nearly as big of an issue for floppies.

Over time, however, a floppy disk can develop difficulties if the track positioning of the actuator drifts from what is normal. This is called a head alignment problem. When the heads become misaligned, you may notice that disks will work if formatted, written and then read in the same drive, but not if moved from one drive to another. This is because the formatting of the floppy is what defines where the data is placed. Misalignment can be solved by having the heads on the floppy disk realigned. This was a common practice when floppy drives cost $500; now that a new disk costs around $30 maximum, nobody realigns regular floppy disk drives, since the realignment labor costs more than a new drive.

The head actuators on a floppy disk are very slow, compared to hard disks, which makes their seek time much higher. While a hard disk's actuator can move from the innermost to outermost tracks (full-stroke seek) in about 20 milliseconds, a floppy disk will typically take 10 times that amount of time or more. This is one reason why floppy disks are much slower than hard disks.

The spindle motor on the floppy is what spins the floppy disk when it is in the drive. When the disk is inserted, clamps come down on the middle of the disk to physically grasp it. These clamps are attached to the spindle motor, which turns the disk as it spins. The speed of the spindle motor depends on the type of floppy drive:

The very slow spindle speeds used for driving floppy disks is another major reason their performance is so poor compared to other media, since the spindle speed affects bothlatency and data transfer rate It is this slow speed however that allows the heads to ride contacting the surface of the media without causing the floppy disk's magnetic coating to wear right off. The spindle motor on a floppy also uses very little power and generates very little heat due to its simple needs.

Modern floppy drives incorporate a special sensor and a signal on the floppy cable that work in conjunction to tell the floppy controller when a disk is ejected and a new one inserted. This signal is used for performance reasons, as keeping track of when the disk is changed also means that the system knows when it hasn't changed. Knowing this saves the system from having to constantly re-examine the disk each time the floppy is accessed to see what's there. Otherwise each time you referenced the disk, the disk's structures would have to be re-examined, causing a great performance penalty.

Occasionally problems can cause the disk-change sensor or circuitry to malfunction, causing strange problems as a result. Typically what happens is that you change the disk but the system doesn't recognize it and thinks the old one is still in there. So when you try to access a file that is on the new disk, it will say the file wasn't found. In reality, it isn't even looking for the file, it is just checking the contents of the last disk that it still has in memory. Also, if you try to write to the new disk you will likely scramble its contents, because the controller will think it is writing the old disk

There are two main connectors that are used to connect the floppy disk to the rest of the PC. There are also jumpers that are used to configure the floppy, however in practice these are never normally touched. The drive select (DS) jumper is used to select whether a floppy is the A: or B: drive in the system, but the convention in the PC is to use the floppy cable to control which is the "A" drive and which is the "B" drive. Virtually all floppies come preconfigured as a "B" drive and the cable determines which one is seen as an "A" drive, instead of the jumpers. Similarly, some drive types have other jumpers to control their operation but they are virtually always left at their defaults.

Note: Some rare systems use SCSI floppy disks, which have different jumpers.

One of the two connectors used to hook up the floppy disk is the power connector. There are two different types used; older 5.25" drives generally use the standard, keyed, 4-wire connectors that are used for hard disks, CD-ROMs and other devices. Most 3.5" drives use the smaller "mini" power connector. It is possible to buy adapters to convert from the larger size to the smaller, but all newer power supplies come with a mini plug specially included to power a 3.5" floppy drive. The power supply chapter discusses this in more detail.

The second connector is the data interface cable. 99.9% of floppy disks use the standard floppy interface, which uses a special 34-pin connector to attach to a floppy interface cable. The cable is important because of a special modification to it that allows it to control the drive letter of the floppies to which it attaches.

There are actually two different types of connectors. The older style uses a card edge connector on the drive; this is usually found on 5.25" drives. The newer style uses a 34-pin header much like the connector for an internal hard disk (but smaller). The latter type of connector is often not properly keyed to prevent incorrect insertion, so this can be a bit of a concern. Usually the indication that a floppy cable has been inserted backwards is the drive not working and the drive busy light coming on and staying on when the machine is booted. In the vast majority of cases reversing the cable will fix the problem and no damage will result to the drive from having been connected the wrong way initially.

The floppy disk contains an integrated logic board that acts as the drive controller. Like the rest of the floppy disk this is a relatively simple affair, containing the electronics that control the read/write heads, the spindle motor, head actuator and other components. The circuits on this board also talk to the floppy disk controller over the floppy interface. SCSI floppy disks of course include a SCSI interface chip on the logic board to talk over the SCSI interface.

The circuit board can be the source of problems on older drives. In theory, the board can be replaced and the drive repaired, but in practice, again, nobody bothers with repair labor on a drive with a replacement value of under $30.

Virtually all floppy disks are internal drives with a faceplate (or bezel) that is used to provide access to the drive door. The size of the faceplate of the drive determines what size drive bay is required for the drive; 5.25" drives go into 5.25" bays, and 3.5" drives can go into 3.5" or 5.25" bays. In the latter case an adapter or mounting kit is required that adapts the drive to the larger bay size. These kits are cheap and you need one if you are putting a 3.5" drive into an older case that doesn't have a 3.5" drive bay.

The height of a 3.5" floppy drive is one inch, matching the height of hard disks which often share the same drive bays. 5.25" drives come in two different heights. The old standard was the so-called full-height drive, which was a whopping 3.5" in height. These are found on only the oldest of PCs and bays for them have not been used for at least a decade. The current standard is the so-called half-height drive, which is (unsurprisingly) about 1.75" in height. This fits into a standard 5.25" drive bay.

The door to the floppy disk drive of course is different for 5.25" drives and 3.5" drives. The former uses a manual latch that when enabled, closes the read/write head and actuator arm on the surface of the disk media. The latter automatically engages the disk media when the disk is inserted, and a button is used to extract the disk when necessary.

Most floppy drives have screw holes tapped into their sides and/or bottom for relatively painless installation. Older systems use drive rails that are used to mechanically slide the drive into the drive bay. Newer systems allow the drives to be screwed into the bays directly.

Since floppy drives use a solid clamping mechanism to hold the disk in place while reading, they can be installed on their sides without any problem. In fact, many desktop cases have their 3.5" drive bay oriented vertically.

Floppy Disk Media and Low-Level Data Structures

Unlike hard disks, which have the recording media integrated into the drive itself--one reason why hard disks are also called "fixed disks"--floppy disks have removable media. Sometimes also called "diskettes", floppy disk media have come a long way in terms of capacity and reliability, and especially price. There was a time when a box of 10 5.25" disks cost $50; now you can get rebate deals at major stores that will end up netting you a box of 50 disks for virtually nothing. This section takes a look at floppy disk media and how it is used by the floppy disk drive.

The first floppy disks were actually not 3.5" or 5.25" at all--they were 8" in size. (And what beasts they are, if you've ever seen them. They are still in use on some very old non-PC equipment.) The 5.25" is the younger cousin of the original floppy and retains, for the mostpart the same basic design as that media, in a smaller size.

The 5.25" disk is comprised of two basic pieces: the actual, round disk media, sometimes called a "cookie", and the protective jacket. The actual disk is made from a thin piece of plastic and is coated with a magnetic material. It has a large hole in its center that is used by the drive to grasp the disk and spin it--the jacket of course does not spin. A slot is cut in the jacket to expose the disk for the read/write heads; it is wide enough for the heads and long enough to allow the actuator to move the heads over all of the tracks on the disk. A notch at the side of the disk acts as a write-protect control; it is somewhat crude however in that you must use tape over the notch to write-protect the disk.

The 5.25" disk earns its name: "floppy". These disks are notoriously fragile. The jacket provides inadequate protection for the disk itself; this, combined with the large size of the disk, makes it very easy to bend. Special care must be taken not to damage them accidentally; basically, they need to be kept inside a plastic box most of the time to avoid destroying them. They do not take kindly to being sent in the mail unless in a larger box. The read/write "window" of the disk is exposed and for this reason the disks can be easily damaged if not kept in their protective paper "pockets". They can even be damaged by writing on the jacket with a ball-point pen, because the jacket is so thin that the pen can cause an impression in the disk media itself.

The lack of durability of the 5.25" media helped contribute to the downfall of the 5.25" floppy disk drive, compared to the 3.5" disks.

3.5" floppy disks are similar in concept of course to 5.25" disks, but offer several improvements in implementation. The three main improvements over the older style of disk all have to do with durability. First, the jacket is made of a much sturdier material that can withstand a reasonable amount of abuse without destroying the disk within. Second, the read/write window of the disk itself is protected by a sliding metal cover that is engaged when the media is inserted into the drive. Finally, the disk itself is smaller, which makes it much sturdier as well.

The 3.5" disk has several other improvements over the 5.25" media as well. The write-protect notch is replaced by a hole with a sliding plastic piece; when the hole is open the disk is write-protected and when it is closed the disk is write-enabled, and switching from one state to the other is simple. The large hole in the center of the 5.25" disk is replaced by a small metal disk with an indexing hole in it, improving durability further.

The 3.5" disk media is reasonably durable and reliable, especially compared to the very flimsy and vulnerable 5.25" media. It doesn't require a protective pocket per se--manufacturers often provide thin plastic pockets with the disks, but I have yet to figure out what exactly the point is of these. The jacket is sturdy enough to handle a ball-point pen with no problems, and they can be mailed readily, as everyone at America Online apparently knows.

The density of the disk surface refers to the amount of data that can be stored in a given amount of space. This is a function of two basic factors: how many tracks can be fit on the disk (track density), and how many bits can be fit on each track (bit density). The product of these two factors is called areal density, normally used to describe the capacity of hard disks; floppy disks are usually instead specified using the separate terms: track density (measured in tracks per inch or TPI) and bit density (measured in bits per inch or BPI).

Virtually every hard disk has a different set of density characteristics. With floppy disks, the densities are more standardized. In fact, there are two standard types of 5.25" disks, and three standard types of 3.5" disks. These vary solely in terms of their density, which are given specific names. This table shows the different density types and their characteristics:

Looking at these numbers you will immediately notice a few things. First, the lowest-density drives strangely use the term "double density". This is because they are successors to even lower-density media that stored even less than they do, many many years ago. Second, even though both 3.5" and 5.25" disks use the terms "double density" and "high density", they refer to entirely different density characteristics. Finally, some densities differ from others based on changes in track density, some based on changes in bit density, and some on both. In particular, all of the 3.5" drives use the same track density, 135 TPI. Double density disks often have the words "double density" printed on them, while high density disks use the familiar "HD" logo, usually stamped into the jacket of the diskette, right near the metal slider.

Some people incorrectly infer from the fact that both low and high density 3.5" disks have the same number of tracks per inch, that the physical disks are interchangeable and the only difference is that the HD disks are "higher quality". This is in fact not the case! As with hard disks, the tighter you pack the data together on a floppy disk, the more the chances of interference between adjacent tracks. Therefore, higher density disks use weaker write signals, and different magnetic coatings than lower density disks.

High density and double density media are not interchangeable. You should always use high-density disks in high-density drives. Considering that the low-density disk is obsolete and the high-density disks are dirt cheap these days, this isn't even something you should think twice about. It is possible to write data to double density disks in a high density drive, but you should always use the correct physical media. While high density drives are downward compatible with double density disks, the high density media is not. If you want to use the 720 KB format you must use double density disks, even if you are using a high density drive. Also, you must issue the correct parameters to the FORMAT command. Typing "FORMAT /?" will show you what they are.

Data encoding is the process of converting binary information (programs and data) into magnetic impulses that can be stored on the magnetic surface of the disk. There are several different ways that this can be done. Floppy disks use the modified frequency modulation (MFM) encoding technique, which is also used for some older hard disks.

There are two steps involved in formatting magnetic media such as floppy disks and hard disks. The first step involves the creation of the actual structures on the surface of the media that are used to hold the data. This means recording the tracks and marking the start of each sector on each track. This is called low-level formatting, and sometimes is called "true formatting" since it is actually recording the format that will be used to store information on the disk.

The second formatting step is high-level formatting. This is the process of creating the disk's logical structures such as the file allocation table and root directory. The high-level format uses the structures created by the low-level format to prepare the disk to hold files using the chosen file system.

For a hard disk, there is an intermediate task that is performed between the two formatting steps: partitioning. For this reason, combined with the incredible complexity of modern hard disks, they are low-level formatted by the manufacturer, and high-level formatting is done by the DOS FORMAT command (or equivalent). Floppy disks require no intermediate step, and due to their relative simplicity, they are both low-level and high-level formatted at the same time by default when you use the FORMAT command.

Once the floppy disk has been low-level formatted, the locations of the tracks on the disk are fixed in place. Since floppies use a stepper motor to drive the head actuator, the floppy drive must be aligned properly in order to read the tracks on the disk. Sometimes the heads of a particular drive can become out of alignment relative to where they should be; when this happens you may notice that a disk formatted on the misaligned drive will work in that drive but not in others, and vice-versa.

Since floppy disks tend to be put together cheaply these days and many of them are getting rather old, it is generally preferable to always low-level format a disk in the drive you plan to use to write to it. If you have a disk that was formatted in another drive that is not working in yours, you can sometimes make it work again by reformatting it. (I personally never bother due to the cost of floppy disks; if I see errors it is gone.)

Tip: Many companies today sell preformatted floppy disks for basically the same cost as unformatted ones. These can save you a great deal of time if you use floppies a lot, and I have rarely encountered problems with them. If you routinely have trouble with good-quality preformatted disks your drive may be out of alignment or need cleaning. Make sure you buy PC formatted disks and not Macintosh ones!

The term geometry refers to the organization of the disk's data structures. For hard disks this is a complicated issue due to the use of physical, logical and translated geometry. For floppy disks it is quite simple: the geometry refers to the number of disk surfaces (which is the same as the number of read/write heads), the number of tracks per surface, and the number of sectors per track. All floppy disks use the same number of sectors for each track, despite the fact that the inner tracks on a disk are much smaller than the outer tracks. This is not the case for hard drives.

Since all (modern) floppy disks use both sides of the disk and therefore always have two heads and surfaces, the only important parameters are the number of tracks and the number of sectors. While hard disks are always referred to as having cylinders, floppies are sometimes said to have cylinders and sometimes to have tracks. Really, they are just different ways of referring to the same thing. Since floppies always have two surfaces, cylinder #N simply refers to track #N on both sides of the disk.

Each floppy disk format (size) has a specific geometry, unlike hard disks which of course vary from disk to disk. Here are the geometry specifications for the different disks formats:

All floppy disks use 512 bytes per sector, which is the standard for both floppies and hard disks. Note that not all of the sectors indicated by the total above are usable for actual user data, because some are used by the file system structures (FAT, root directory etc.).

Floppy Disk Formats and Logical Structures

There are several different disk formats that are in common use on the PC platform. Actually, "common use" is debatable, because really only one format is used any more, and even it has limited uses these days. This section takes a look at the different floppy disk formats used for the various types of drives. It also looks at how the FAT file system is implemented on floppy disks.

The oldest floppy disk format is the 360 KB 5.25" floppy disk. This is the type of disk that was used in the very first IBM PCs, which in fact didn't use a hard disk at all. The 360 KB floppy is the only format that uses 40 tracks per side to record data; the others all use 80 tracks. This accounts for its low capacity. Older versions of these drives were full-height models; the half-height models were introduced later.

The 360 KB drive is thoroughly and completely obsolete, having been replaced by the 1.2 MB 5.25" floppy disk. If you still have one of these in your system you really need to think about upgrading your hardware.

The high density 1.2 MB floppy disk debuted in the IBM AT in 1984, as a standard feature (the 360 KB floppy was optional). The increase in capacity of this disk, over 200% compared with the 360 KB version, all but obsoleted the smaller format rather quickly. The 1.2 MB floppy disk can still read and write 360 KB floppies, but problems can occasionally result. Since the floppy uses a higher bit density, the 1.2 MB floppy requires a floppy disk controller capable of 500 Kbits/s data transfer. Virtually all newer controllers support this rate.

At the time that this disk was introduced, it was considered relatively "roomy". Compared to a 20 MB hard disk, a 1.2 MB floppy is pretty decent storage. The 5.25" disk has all but died out due to the physical advantages of the 3.5" formats. Still, some older PCs continue to use their 1.2 MB disks to this day.

The original version of the 3.5" floppy disk held 720 KB of data and was introduced in 1986. This version of the 3.5" never became very popular both because it offered 40% less capacity than the 1.2 MB 5.25" drive, and because it was so quickly replaced by the high density 3.5" disks.

The oldest PCs, from before around 1985, do not support the 3.5" floppy drive due to limitations in their internal BIOS.

The only floppy disk format still in wide use, the 1.44 MB 3.5" drive was introduced by IBM in 1987 as part of its PS/2 line. Since that time they have grown immensely popular, and this format is the standard for floppy disks today. Virtually every PC made since 1987 uses one of these drives, and there are many non-PC computers that will read them as well. They have become very cheap due to the aging of the technology and the fact that they are produced in such high volume. Their universality is what has allowed the floppy to continue to be a default part of every PC despite their rather tiny storage capacity compared to today's hard disks. Since the 1.44 MB uses a higher bit density than the 720 KB, it requires a floppy disk controller capable of 500 Kbits/s data transfer. Virtually all newer controllers support this rate.

The 1.44 MB floppy disk will read and write 720 KB disks, but you should always use the correct density of media when writing to these disks

The highest-capacity format for floppy disks is the 2.88 MB 3.5" disk that was developed by Toshiba in the late 80s. The 2.88 MB offers double the capacity of the 1.44 MB disk by using special media and a special recording method.

If you've never used or even heard of this drive format before, then you are exactly right. It is basically a dead format; I very rarely if ever see it used. The exact reason why is anybody's guess, but these three factors weigh heavily in my mind when I consider why it might be that nobody uses 2.88 MB floppies despite the fact that everyone hates the small size of the 1.44 MB floppy:

• Inertia: By the time the 2.88 MB went into full production, the PC explosion was well underway with a very large number of these existing machines using 1.44 MB disks. The 1.44 was the standard and so people were reluctant to use 2.88 MB disks because of the installed base of PCs that could not read them.
• Compatibility: The very high bit density of the 2.88 MB floppy--36 sectors per track--means that a floppy controller capable of 1 Mbits/s transfer rate is required. Many older PCs only had a controller capable of 500 Kbits/s transfers. Using the newer drive would have required a new controller card. (Virtually all new PCs are capable of supporting the 2.88 MB disk with no problem.)
• Media Cost: The media for these drives is very expensive. In fact, right now, 1.44 MB disks can be had for less than 50 cents each, and in some cases for free by using mail-away rebates. In contrast, 2.88 MB disks still cost several dollars a piece! Now, this is a bit of a chicken-and-egg thing, because if the disks aren't popular the production volume is low, and this will make the cost high, thus making the disks even less popular, and so on. However, it was probably a significant contributing factor to the 2.88 MB not catching on.

Of course the old standbys of "poor marketing" and "overpriced hardware" could have been contributing factors as well.

Generally speaking, floppies use the FAT file system. This is the basic file system used by DOS, Windows 3.x, Windows 95 and optionally on Windows NT. The FAT file system is described in great detail in the file system section on hard disks; you will probably want to refer there if you want to learn more about FAT and how it works. I'll talk a bit here about some floppy-specific aspects of how FAT is used.

Floppy disks use the same basic structures as hard disks, only they are less complicated. Since floppies need not (and cannot) be partitioned, they do not have partition tables or a master boot record. Conceptually, a floppy disk has the same structures as a single hard disk volume. The disk has a single volume boot sector for the disk, and this is what is used when a floppy is booted (assuming the floppy is bootable).

All floppy disks are formatted in the FAT12 version of the file system; this is the oldest flavor of FAT and is sufficient for the small capacity of floppies. The cluster size of floppy disks is either one or two sectors, depending on the disk type. I am not really sure why, but the larger disks use smaller clusters; go figure. Either way, the clusters are small which means that the space on the disk is effectively utilized; there is little slack on a floppy disk, since even a two-sector cluster is only 1,024 bytes in size.

Floppies also have serious limitations on the number of entries in their root directory, far less than the 512 entries in the root directory of a hard disk. Again, this depends on the type of disk:

Formatted and Unformatted Capacity

Floppy disks often have two capacity specifications; they are often quoted with both their unformatted capacity, and their formatted capacity. Since the disk is useless unless it is formatted (well, other than as a coaster for your coffee table) the unformatted capacity means basically nothing. The formatted capacity is the true maximum capacity of the disk. Usually, the formatted capacity is about three-quarters the unformatted capacity.

Even the formatted capacity, however, doesn't show the true amount of space available for user files, because a certain amount of overhead is taken up for FAT file structures. This is true of hard disks as well, of course, although as a percentage more of the floppy is taken up by this information than a hard disk is. The amount of space remaining after these structures are placed on the disk is the true usable capacity of the floppy.

Note also that the "decimal vs. binary" measurements problem is in play again with the terminology used to specify floppy capacity. In fact, the terms are not even consistent in and of themselves. For example, a 1.44 MB floppy disk takes its name from the fact that the disk has 2,880 sectors, and each sector is 0.5 KB; 0.5 times 2,880 is 1,440, so the 1.44 is a decimal measure. But, each sector is really 512 bytes, so the 0.5 KB is a binary measure. As a result the "1.44" is a mixed measurement; the true raw formatted capacity is either 1.41 MB (binary) or 1.47 MB (decimal), and not 1.44 MB at all!

Here are the capacity figures for the various disk types:

The following table shows a summary of the various floppy disk specifications provided in other sections of this chapter, for each of the five major floppy disk types:

Floppy Disk Interfacing and Configuration

The floppy disk controller is the piece of hardware responsible for interfacing the floppy disk drives in your PC to the rest of the system. It manages the flow of information over the interface and communicates data read from the floppies to the system processor and memory, and vice-versa. This section discusses various issues related the control, the floppy disk interface, and floppy drive configuration.

Many people don't realize this but special floppy disks exist that are interfaced over a SCSI bus. This setup is very rarely used in PCs but is sometimes found outside the PC world and also in specialized industrial applications. The floppy disk is attached to a SCSI host adapter using a special cable. Of course, this arrangement requires a host adapter that supports floppy disks; not all of them do.

Modern PCs generally do not bother with SCSI floppy drives, even the ones that use SCSI for hard disks and other peripherals. Since the floppy disk is not a primary medium for storage or even in most cases, a primary means of data transfer, it is more cost effective to just use a cheap regular floppy disk drive instead of purchasing an expensive SCSI floppy. After all, the floppy disk controller is built into the motherboard anyway.

Regular floppy disks use their own interface, usually called the floppy disk interface for obvious reasons. This interface is derived from older non-PC designs that go back more than two decades. The floppy interface is a very simple affair for this reason, and in today's PC there isn't really much to say about it. Unlike hard disk interfaces where there are compatibility and performance issues galore, with floppy disks it is generally either "it works" or "it doesn't", and usually, "it works".

Over time, some other devices have adopted the floppy disk interface, such as tape backup drives.

At one time the floppy controller was a dedicated card inserted into an expansion slot in the motherboard. Later, floppy controllers were placed onto multifunction controller cards that also provided IDE/ATA hard disk interfacing, and serial and parallel ports. These cards are commonly found on ISA-based and VLB-based PCs created from around 1990 to 1994.

Since floppy disk controllers have basically not changed in quite some time, it has been possible to standardize and miniaturize them. The latest Pentium-class and later motherboards using PCI bus architecture, almost always include floppy disk controllers right on the motherboard. This support is usually provided through the use of a Super I/O chip that includes support for the floppies, serial/parallel ports and occasionally IDE/ATA hard disks.

The floppy disk controller included in virtually all new PCs will support every type of standard floppy disk. Older controllers, however, would not work with the newer drives. Generally speaking, the limiting factor was the floppy controller's ability to run at a high enough speed. While the floppy interface is in general very slow--far slower than hard disk interfaces, even at the floppy's top speed--there are in fact different "shades" of slow.

The speed required of the controller is directly related to the density of the floppy disk media being used, in particular the bit density per track. Since higher-density floppies record more information in the same space, they require faster data transfer to the drive, to ensure that the data arrives "on time" to be recorded. There are currently three different controller speeds:

• 250 Kbits per Second: The slowest speed controller, this type will only support the lowest of the floppy densities; this means the double-density 360 KB 5.25" and 720 KB 3.5" drives. This type of controller is thoroughly, completely obsolete.
• 500 Kbits per Second: Found on a large number of PCs, the 500 Kbits controller will support all disks except for the 2.88 MB 3.5". This type of controller has been used for many years, although it has in the last few years been mostly replaced by the 1 Mbits/s controller.
• 1 Mbits per Second: The most modern type, this controller will support all of the floppy disk formats on the market.

Today, the speed of the floppy controller is actually more important when dealing with floppy interface tape drives. In many cases using the full capacity of the tape drive is dependent upon the floppy controller being fast enough to handle the high data transfer rates required by the latest tape formats. This is actually the same exact reason that causes high-capacity floppy formats not to be supported by older, slower controllers.

Tip: You can usually tell what sorts of floppy drives your system will support based on the allowable entries in the floppy drive setup BIOS parameters.

Floppy disks are so universal that their resource usage has been all but standardized and has become quite universal as well. In fact, most peripherals won't even let you select as options the resources that are normally used by the floppy disk controller, knowing that in virtually every PC these resources are not available.

The floppy disk controller on a standard PC uses the following resources:

• Interrupt Request Line (IRQ): The standard floppy controller IRQ is 6. This is occasionally given as an option for tape accelerator cards, because they also use the floppy interface protocol and can be used as replacements for the standard floppy controller. Virtually all other peripherals will avoid IRQ 6.
• DMA Channel: The standard direct memory access (DMA) channel for the floppy controller is 2. Again, tape accelerator cards offer this is an option while most other peripherals avoid it.
• I/O Address: The standard I/O address range for the standard floppy controller is 3F0-3F7h. Strangely, this overlaps with the standard I/O address for the slave device on the primary IDE channel. In this case, however, this does not constitute a resource conflict because the overlap is something that has existed for a very long time and is compensated for. It is not an issue of concern.

In some cases these resources can be changed; in most cases they cannot. Even when they can be changed, they very rarely are since they really are the standard.

The floppy disk interface uses what is considered, by most people, a truly strange cable. It is similar to the standard IDE cable in that it is usually a flat, gray ribbon cable. It is unusual in terms of the number of connectors it has and how it is used to configure the setup of the floppy disks in the system.

The floppy cable has 34 wires. There are normally five connectors on the floppy interface cable, although sometimes there are only three. These are grouped into three "sets"; a single connector plus two pairs of two each (for a standard, five-connector cable) or three single connectors. This is what they are used for:

• Controller Connector: The single connector on one end of the cable is meant to connect to the floppy disk controller, either on a controller card or the motherboard.
• Drive A Connectors: The pair of connectors (or single connector in the case of a three-connector cable) at the opposite end of the cable is intended for the A: floppy drive. This is explained in more detail below.
• Drive B Connectors: The pair of connectors (or single connector in the case of a three-connector cable) in the middle of the cable is intended for the B: floppy drive.

The reason that the standard cable uses pairs of connectors for the drives is for compatibility with different types of drives. 3.5" drives generally use a pin header connector, while 5.25" drives use a card edge connector. Therefore, each position, A and B, has two connectors so that the correct one is available for whatever type of floppy drive being used. Only one of the two connectors in the pair should be used (they're too close together to use both in most cases anyway). The three-connector cables are found either in very old systems or in ones where the manufacturer was trying to save a few pennies. They reduce the flexibility of the setup; fortunately these cables can be replaced directly by the five-connector type if necessary.

You will also notice that there is an odd "twist" in the floppy cable, located between the two pairs of connectors intended for the floppy drives. Despite the fact that this appears to be a "hack" (well, it really is a hack), this is in fact the correct construction of a standard floppy interface cable. There are some cables that do not have the twist, and it is these that are actually non-standard! What the twist does it to change the connection of the drive on the far end of the twist so that it is different than the drive before the twist. This is done to cause the drive at the end of the cable to appear as A: to the system and the one in the middle to be as B:.

Here's how it works in detail. Traditionally, floppy drives used a drive select (DS) jumper to configure the drive as either A: or B: in the system. Then, special signals were used on the floppy interface to tell the two drives in the system which one the controller was trying to talk to at any given time. The wires that are cross-connected via the twist are signals 10 to 16 (seven wires). Of these, 11, 13, and 15 are grounds and carry no signal, so there are really four signals that are inverted by the twist. The four signals that are inverted are exactly the ones that control drive selection on the interface. Here is what happens when the twisted cable is used:

Since the signals are inverted, the drive after the twist responds to commands backwards from the way it should; if it has its drive select jumpers set so that it is an A: device, it responds to B: commands, and vice-versa.

Why on earth would anyone have wanted to do this? Basically, because it was a big time-saver during setup back in the days when it was quite common to find two floppy drives in a machine. Without the twist, if you wanted to use two floppy drives one had to be jumpered as A: and the other as B:. With the twist, you just leave them both jumpered as B:, and whichever one you put after the twist will appear to the system as A: because the control lines are inverted. If you want to change the setup so that the other drive is A: instead, you just switch the cable. If you only want one drive, you only use the connector after the twist. Large manufacturers, therefore, could arrange to have all of their floppy disks configured the same way without having to pull jumpers as the PC was assembled.

In order for this system to work, both drives must be jumpered as B: drives. Since the floppy cable with the twist is standard, this jumpering scheme has become the standard as well. Virtually all floppy disks that you purchase come prejumpered as B: drives so that they will work with this setup.

If this whole setup sounds like it is the same thing as the cable select protocol for IDE/ATA hard disks, that's because it is basically the same idea. IDE/ATA hard disks require you to change themaster/slave jumpers in a similar manner, and cable select was invented to do away with this. The difference is, as usual, just one of inertia and history; the floppy drive system is the standard while cable select never caught on for hard disks.

Tip: Some newer BIOSes have taken things a step further. They include a BIOS parameter that will invert the A: and B: signals within the controller itself. When enabled, this lets you reverse whichever drive is A: with the one that is B:, without requiring you to even open the case. Note however that this is not compatible with all operating systems: in particular, both Windows NT and Linux can malfunction with this swap feature set, which can cause serious problems when trying to install the operating system. The reason this happens is that the swap setting only affects the way the BIOS handles the floppy drive, and confuses operating systems that go directly to the hardware.

Note: Apparently, there is yet another floppy cable variant out there, that is used by some manufacturers. In this setup, there are actually two twists in the floppy cable. The drive placed after the first twist, in the middle of the cable, is A:, much as it is with the standard one-twist cable. The drive placed after the second twist is B:. The second twist "reverses" the effect of the first one and makes the connector at the end of the cable operate the same way a drive that appears before the twist in a regular cable does. I have no idea what the benefits of using a cable like this are over a standard floppy cable.

Virtually all PCs support two floppy drives through the standard BIOS setup. The standard BIOS setup screen provides access to the two setup parameters where you tell the system what types of floppy drives are in the system. Support for different drive types is generally based on support from the controller, and the BIOS parameters are arranged so that you can only select the drives that the controller can handle.

In addition, there are a couple of other BIOS parameters that are directly related to the floppy drives:

• Boot Sequence: This parameter controls the order in which the BIOS will try to boot the system.
• Floppy Drive Seek: Some BIOSes allow you to disable the seek that the system does to verify the presence of the floppy drive(s) at boot time.
• Swap Floppy Drives: This parameter switches which disk is viewed as A: and which is B: by the system.

Configuring floppy disks that use the standard floppy interface is quite straightforward. Since there can only be one floppy interface, and only two drives on the interface, configuration is simply a matter of making sure each drive has its power connector plugged in securely, and then connecting the floppy cable to each device. The connector on the floppy cable used to attach to each device controls which one appears as A: and which as B:.

Many problems with floppy drives relate to cables that come loose or are inserted backwards. A drive with the cable in backwards will not function correctly and often, the activity LED will come on and stay on when the PC is booted.

In some ways, the terms "floppy disk" and performance don't belong in the same sentence. That sounds a bit harsh, but really, the floppy disk is the slowest storage device in your PC. At least I sure hope it is for your sake! Since the floppy drive is standardized, and the controllers are standardized, there is nothing any regular PC user can do to improve hardware performance when it comes to floppies. Therefore, there isn't much to talk about in this regard.

There actually is one legitimate performance factor when it comes to floppy disks, but it is actually more of an operating system issue than a hardware issue. Traditionally, floppy disks have been accessed using standard BIOS routines and this has contributed to their rather slow performance. Furthermore, their use in a multitasking environment has been quite poor. If you've ever used Windows 3.x and tried to do anything at the same time that you were doing any work with the floppy disk, you probably noticed how the entire system would seem to slow to a crawl.

Under 32-bit operating systems like Windows 95 and Windows NT, floppy drive access is performed using 32-bit protected mode drivers. This is similar to the way that these operating systems access hard disks for improved performance. The difference in floppy speed under Windows 95, compared to DOS for example, is actually quite striking; it's very noticeable. And you can use the floppy disk while performing other tasks.

The reliability of floppy disk media is a bit of a sore spot for me. While the drives are generally quite reliable and will last for many years if given even a modicum of reasonable care, the media are in fact quite unreliable in my experience. Certainly the condition of the drive will contribute to this; an older drive that is dirty and misaligned will cause many more problems than a newer drive that is clean and aligned properly. Even so, failure of floppy disk media is more a matter of when than if. This is true of hard disks too but the time frame is much smaller here; usually I find that floppy disks that I use regularly will develop bad sectors (unreadable areas on the disk) within a few weeks, and often in less time than that.

There are several reasons why floppy disks are far less reliable, in general, than hard disks for example. One is quality in general--most floppy disk drives are crude affairs assembled in large quantity and sold very cheaply; it's hard to have very good quality in a drive that sells for $25. Floppy disk media is in many cases even worse; competition among manufacturers is frequently based on cost only, since the media is viewed as a commodity item and not something for which quality matters.

The nature of floppy disk technology contributes to low reliability as well. While hard disks have the complex task of dealing with a very fast spinning disk and read/write heads floating very near to them on a cushion of air, they do this in a tightly controlled environment. The head assembly is sealed, and the platters are fixed and rigid. Floppy disks use removable media that is not rigid, and both the heads and media are exposed to external contaminants that can damage the media and lead to data loss. Data stored on floppy disks is also subject to loss as a result of stray magnetic fields.

As far as I am concerned, floppy disks are reliable only for short-term storage and data transfers. I would not attempt long-term archiving on floppy disks, and I no longer view them as a viable backup source for critical data (they're too small anyway). When I am doing data transfers using floppies, I almost always make two copies of the file on two floppies in case one goes bad.

Warning: I've seen too many people that actually do original work on floppy disks, and then are surprised when one day, the file won't read from the disk. In my opinion anyone that uses floppy disks for primary storage is asking for trouble (and will usually get it.) It makes much more sense to use the hard disk and then copy to the floppy disk when you are finished. It's much faster as well--saving a large document from a typical application to a floppy disk takes much more time than saving it to the hard disk and then copying it to the floppy, because of the non-linear way that the application saves the data.

Finally, if I get a bad disk, I immediately toss it (I never arrange to have critical data on a floppy but if I did I would recover as much data as I could, and then toss it). It isn't worth trying to "save" a floppy disk that is showing errors, in my experience, because they will usually start showing more errors, and because they are so cheap anyway. Repeated problems with the same drive of course implicate the drive (or the media in general if you are always using the same brand).

Due to its universality, the floppy interface has been used by non-floppy-disk peripherals in the past. However, due to its slow speed, the number of devices that use this interface is quite small.

The most widely encountered peripherals that use the floppy disk interface are tape drives, in particular QIC or Travan drives. These are inexpensive, quarter-inch cartridge (QIC) tape backup units, that make use of the floppy disk interface since they don't require a very great amount of speed. Most of the newer types of drives require the fast 1 Mbits/second controller speed, as described here.