Upgrading and Repairing PCs

Disk Physical Specifications and Operation

Most PCs sold today are equipped with a 3 1/2'' 1.44MB floppy disk drive. In rare cases, you might run into a very old system that has a 5 1/4'' 1.2MB drive instead of, or in addition to, the 3 1/2'' drive. Plus, a few older PC systems have a 2.88MB 3 1/2'' drive that can also read and write 1.44MB disks. The older drive types—5 1/4'' 360KB and 3 1/2'' 720KB—are obsolete and rarely found anymore.

The physical operation of a disk drive is fairly simple to describe. The disk rotates in the drive at either 300rpm or 360rpm. Most drives spin at 300rpm; only the 5 1/4'' 1.2MB drives spin at 360rpm. With the disk spinning, the heads can move in and out approximately one inch and write 80 tracks. The tracks are written on both sides of the disk and are therefore sometimes called cylinders. A single cylinder comprises the tracks on the top and bottom of the disk. The heads record by using a tunnel-erase procedure that writes a track to a specified width and then erases the edges of the track to prevent interference with any adjacent tracks.

Different drives record tracks at different widths. Table 11.4 shows the track widths in both millimeters and inches for the various types of floppy disk drives found in modern PC systems.

Table 11.4. Floppy Disk Drive Track-Width Specifications

Drive Type	No. of Tracks	Track Width
5 1/4'' 360KB	40 per side	0.300mm	0.0118''
5 1/4'' 1.2MB	80 per side	0.155mm	0.0061''
3 1/2'' 720KB	80 per side	0.115mm	0.0045''
3 1/2'' 1.44MB	80 per side	0.115mm	0.0045''
3 1/2'' 2.88MB	80 per side	0.115mm	0.0045''

How the Operating System Uses a Disk

To the operating system, data on your PC disks is organized in tracks and sectors, just as on a hard disk drive. Tracks are narrow, concentric circles on a disk; sectors are pie-shaped slices of the individual tracks.

Table 11.5 summarizes the standard disk formats for PC floppy disk drives.

Table 11.5. 5 1/4'' and 3 1/2'' Floppy Disk Drive Formats

5 1/4'' Floppy Disks
	Double-Density 360KB (DD)	High-Density 1.2MB (HD)
Bytes per sector	512	512
Sectors per track	9	15
Tracks per side	40	80
Sides	2	2
Capacity (kilobinary)	360	1,200
Capacity (megabinary)	0.352	1.172
Capacity (million bytes)	0.369	1.229
3 1/2'' Floppy Disks
	Double-Density 720KB (DD)	High-Density 1.44MB (HD)	Extra High-Density 2.88MB (ED)
Bytes per sector	512	512	512
Sectors per track	9	18	36
Tracks per side	80	80	80
Sides	2	2	2
Capacity (kilobinary)	720	1,440	2,880
Capacity (megabinary)	0.703	1.406	2.813
Capacity (million bytes)	0.737	1.475	2.949

You can calculate the capacity differences between various formats by multiplying the sectors per track by the number of tracks per side together with the constants of two sides and 512 bytes per sector. Note that the floppy disk's capacity can actually be expressed in various ways. For example, what we call a 1.44MB disk really stores 1.475MB if you go by the correct decimal prefix definition for megabyte. The discrepancy comes from the fact that in the past floppies were designated by their kilobinary (1024-byte) capacities, which in the past were abbreviated KB. The current abbreviation for kilobinary, however, is KiB according to the International Electrotechnical Commission (IEC).

Despite the IEC standards, the traditional method when discussing floppy drives or disks is to refer to the capacity of a floppy by the number of kilobinary bytes (1,024 bytes equals 1KiB) but to use the otherwise improper abbreviation KB instead. This also has been improperly extended to the abbreviation MB. Therefore, a disk with an actual capacity of 1,440KiB is instead denoted as a 1.44MB disk, even though it would really be 1.406MiB (megabinary bytes) or 1.475MB (million megabytes) if we went by the correct definitions for MiB (mebibyte) and MB (megabyte).

For the remainder of this chapter, I will refer to the capacity of the various floppy disks according to the previously used conventions rather than the more technically accurate IEC-designated binary and decimal prefixes.

Note Again, as with hard disk drives, both megabyte and millions of bytes have been improperly abbreviated as MB or M, often resulting in a great deal of confusion. The IEC prefixes for binary multiples was designed to eliminate this confusion. For more information on the 1998 IEC prefixes for binary multiples, see http://physics.nist.gov/cuu/Units/binary.html. Also, see the section "Capacity Measurements" in Chapter 9.

Like blank sheets of paper, new, unformatted disks contain no information. Formatting a disk is similar to adding lines to the paper so you can write straight across. Formatting the disk writes the information the operating system needs to maintain a directory and file table of contents. On a floppy disk, no distinction exists between a high-level and low-level format, nor do you have to create any partitions. When you format a floppy disk with Windows Explorer or the command prompt FORMAT.COM, both the high- and low-level formats are performed simultaneously.

When you format a floppy disk, the operating system reserves the track nearest to the outside edge of a disk (track 0) almost entirely for its purposes. Track 0, Side 0, Sector 1 contains the Volume Boot Record (VBR), or Boot Sector, that the system needs to begin operation. The next few sectors contain the file allocation tables (FATs), which keep records of which clusters or allocation units on the disk contain file information and which are empty. Finally, the next few sectors contain the root directory, in which the operating system stores information about the names and starting locations of the files on the disk.

Note that most floppies today are sold preformatted. This saves time because the formatting can take a minute or more per disk. Even if disks come preformatted, they can always be reformatted later.

Cylinders

The cylinder number is normally used in place of the track number because all floppy drives today are double-sided. A cylinder on a floppy disk includes two tracks: the one on the bottom of the disk above Head 0 and the one on the top of the disk below Head 1. Because a disk can't have more than two sides and the drive has two heads, there are always two tracks per cylinder for floppy disks. Hard disk drives, on the other hand, can have multiple disk platters—each with two heads—resulting in many tracks per single cylinder. The simple rule is that there are as many tracks per cylinder as there are heads on the drive.

Cylinders are discussed in more detail in Chapter 10, "Hard Disk Storage," and Chapter 24, "File Systems and Data Recovery."

Clusters or Allocation Units

A cluster also is called an allocation unit. The term is appropriate because a single cluster is the smallest unit of the disk that the operating system can allocate when it writes a file. A cluster or an allocation unit consists of one or more sectors—usually a power of two (1, 2, 4, 8, and so on). Having more than one sector per cluster reduces the FAT size and enables the OS to run more quickly because it has fewer individual clusters to manage. The tradeoff is in some wasted disk space. Because the OS can manage space only in the cluster size unit, every file consumes space on the disk in increments of one cluster.

For more information on allocation units, see Chapter 24.

Table 11.6 lists the default cluster sizes used for various floppy disk formats.

Table 11.6. Default Cluster and Allocation Unit Sizes

Floppy Disk Capacity	Cluster/Allocation Unit Size	FAT Type
5 1/4'' 360KB	2 sectors	1,024 bytes	12-bit
5 1/4'' 1.2MB	1 sector	512 bytes	12-bit
3 1/2'' 720KB	2 sectors	1,024 bytes	12-bit
3 1/2'' 1.44MB	1 sector	512 bytes	12-bit
3 1/2'' 2.88MB	2 sectors	1,024 bytes	12-bit
KB = 1,024 bytes (by convention) MB = 1,000 KB bytes (by convention)

Disk Change

The standard PC floppy controller and drive use a special signal on pin 34 called Disk Change to determine whether the disk has been changed—or more accurately, to determine whether the same disk loaded during the previous disk access is still in the drive. Disk Change is a pulsed signal that changes a status register in the controller to let the system know that a disk has been either inserted or ejected. This register is set to indicate that a disk has been inserted or removed (changed) by default.

The register is cleared when the controller sends a step pulse to the drive and the drive responds, acknowledging that the heads have moved. At this point, the system knows that a specific disk is in the drive. If the Disk Change signal is not received before the next access, the system can assume that the same disk is still in the drive. Any information read into memory during the previous access can therefore be reused without rereading the disk.

Because of this process, systems can buffer or cache the contents of the FAT or directory structure of a disk in the system's memory. By eliminating unnecessary rereads of these areas of the disk, the apparent speed of the drive is increased. If you move the door lever or eject button on a drive that supports the Disk Change signal, the DC pulse is sent to the controller, thus resetting the register and indicating that the disk has been changed. This procedure causes the system to purge buffered or cached data that had been read from the disk because the system then can't be sure that the same disk is still in the drive.

One interesting problem can occur when certain drives are installed in a 16-bit or greater system. As mentioned, some drives use pin 34 for a "Ready" (RDY) signal. The RDY signal is sent whenever a disk is installed and rotating in the drive. If you install a drive that has pin 34 set to send RDY, the system thinks it is continuously receiving a Disk Change signal, which causes problems. Usually, the drive fails with a Drive Not Ready error and is inoperable. The only reason the RDY signal exists on some drives is that it happens to be a part of the standard Shugart SA400 disk interface; however, it has never been used in PC systems.

The biggest problem occurs if the drive should be sending the DC signal on pin 34 but isn't. If a system is told (through CMOS setup) that the drive is any type other than a 360KB (which can't ever send the DC signal), the system expects the drive to send DC whenever a disk has been ejected. If the drive is not configured properly to send the signal, the system never recognizes that a disk has been changed. Therefore, even if you do change the disk, the system still acts as though the first disk is in the drive and holds the first disk's directory and FAT information in RAM. This can be dangerous because the FAT and directory information from the first disk can be partially written to any subsequent disks written to in the drive.

If the drive you are installing is a 5 1/4'' 1.2MB or 3 1/2'' 720KB, 1.44MB, or 2.88MB drive, be sure to set pin 34 to send the Disk Change (DC) signal. Most drives come permanently preset this way, but some have used a jumper (usually labeled DC) to set this option.

Caution If you have ever seen a system with a floppy disk drive that shows "phantom directories" of the previously installed disk, even after you have changed or removed it, you have experienced this problem firsthand. The negative side effect is that all disks after the first one you place in this system are in extreme danger. You likely will overwrite the directories and FATs of many disks with information from the first disk. If it's even possible at all, data recovery from such a catastrophe can require quite a bit of work with utility programs, such as Norton Utilities. These problems with Disk Change most often are traced to an incorrectly configured drive. Note that Windows Explorer doesn't always display the new contents of a drive. Press the F5 key to refresh the display after you change floppy disks to force the computer to read the new disk.