TOPIC 2—FLOPPY DISKS AND DISK DRIVES

The date and time fields are updated every time the file is changed. The starting cluster field indicates where the beginning of the file is stored on the disk. DOS uses clusters to define disk areas. Depending on the type and capacity of a disk, a cluster can be from 1 to 128 sectors. A 5.25-inch, 360K floppy disk has 2 sectors per cluster. A 32M freed disk has 4 sectors per cluster. DOS uses the starting cluster field to reference the file allocation table (FAT) to get information as to where the entire file is stored. File Allocation Table The file allocation table (FAT) is created during the formatting process. There is a FAT entry for each cluster on the disk. A FAT entry will be — a zero (0), to indicate the cluster is available for storage, — an end of file code, — a bad cluster code (written during formatting), or — a number that points to the next cluster in the file. Suppose we have a file named EVAL.ABC on a 5.25-inch, 360K floppy disk. The file is 4,608 bytes long and could be stored in 4.5 clusters. DOS cannot use partial clusters so this file would occupy 5 full clusters. The directory entry for the starting cluster indicates cluster 25 as the first cluster of this file. Table 10-3 illustrates what the FAT entries for this file might look like. As illustrated in table 10-3, the disk had clusters 25, 26, and 27 available to store EVAL.ABC, then had more data so the rest of the file was stored in clusters 70 and 71. Note also the FAT is a one-way pointer. That is, by examining the contents of the entry for cluster 70, we see that the file continues in cluster 71, but we don’t know the previous cluster was cluster 27. TOPIC 2—FLOPPY DISKS AND DISK DRIVES Floppy disks are available in a variety of densities for each size of disk. The disks are labeled as to the maximum density each is designed to handle. Table 10-4 shows the sizes and densities of some floppy disks. The differences between the disk types listed in tables 2-4 and using them in various drives is covered in detail later in this chapter. Table 10-3.—Contents of a File Allocation Table Table 10-4.—Floppy Disk Density Formats 10-6