Upgrading and Repairing PCs

Types of Floppy Disk Drives

The characteristics of the floppy disk drives you might encounter in PC-compatible systems are summarized in Table 11.7. As you can see, the different disk capacities are determined by several parameters, some of which seem to remain constant on all drives, although others change from drive to drive. For example, all drives use 512-byte physical sectors, which is true for hard disks as well.

Table 11.7. Floppy Disk Logical Formatted Parameters

	Current Formats					Obsolete Formats
Disk Size (Inches)	3 1/2	3 1/2	3 1/2	5 1/4	5 1/4	5 1/4	5 1/4	5 1/4
Disk Capacity (KB)	2,880	1,440	720	1,200	360	320	180	160
Media descriptor byte	F0h	F0h	F9h	F9h	FDh	FFh	FCh	FEh
Sides (heads)	2	2	2	2	2	2	1	1
Tracks per side	80	80	80	80	40	40	40	40
Sectors per track	36	18	9	15	9	8	9	8
Bytes per sector	512	512	512	512	512	512	512	512
Sectors per cluster	2	1	2	1	2	2	1	1
FAT length (sectors)	9	9	3	7	2	1	2	1
Number of FATs	2	2	2	2	2	2	2	2
Root dir. length (sectors)	15	14	7	14	7	7	4	4
Maximum root entries	240	224	112	224	112	112	64	64
Total sectors per disk	5,760	2,880	1,440	2,400	720	640	360	320
Total available sectors	5,726	2,847	1,426	2,371	708	630	351	313
Total available clusters	2,863	2,847	713	2,371	354	315	351	313

1.44MB 3 1/2'' Drives

The 3 1/2'', 1.44MB, high-density (HD) drives first appeared from IBM in the PS/2 product line introduced in 1987. Most other computer vendors started offering the drives as an option in their systems immediately afterward. This type of floppy drive is still the most popular in systems today.

The drive records 80 cylinders consisting of 2 tracks each with 18 sectors per track, resulting in a formatted capacity of 1.44MB. Some disk manufacturers label these disks as 2.0MB, and the difference between this unformatted capacity and the formatted usable result is lost during the format. Note that the 1440KB of total formatted capacity does not account for the areas the FAT file system reserves for file management, leaving only 1423.5KB of actual file-storage area.

The drive spins at 300rpm and in fact must spin at that speed to operate properly with existing high- and low-density controllers. To use the 500KHz data rate (the maximum from most standard high- and low-density floppy controllers), these drives must spin at a maximum of 300rpm. If the drives were to spin at the faster 360rpm rate of the 5 1/4'' drives, they would have to reduce the total number of sectors per track to 15; otherwise, the controller could not keep up. In short, the 1.44MB 3 1/2'' drives store 1.2 times the data of the 5 1/4'' 1.2MB drives, and the 1.2MB drives spin exactly 1.2 times faster than the 1.44MB drives. The data rates used by both of these HD drives are identical and compatible with the same controllers. In fact, because these 3 1/2'' HD drives can run at the 500KHz data rate, a controller that can support a 1.2MB 5 1/4'' drive can also support the 1.44MB drives.

Other Floppy Drive Types

Other types of floppy drives that have been used in the past include the following:

• 2.88MB 3 1/2''. This size was used on some IBM PS/2 models in the early 1990s.

• 720KB 3 1/2''. This size was used by IBM and others starting in 1986 before the 1.44MB 3 1/2'' was introduced.

• 1.2MB 5 1/4''. It was introduced by IBM for the IBM AT in 1984 and widely used throughout the rest of the decade.

• 360KB 5 1/4''. An improved version of the floppy disk drive originally used by the IBM PC, it was used throughout the 1980s on XT-class machines and some AT-class machines.

If you need more information about these drives, see the Technical Reference section of the DVD included with this book.