Directories

Electronics Technician Volume 06-Digital Data Systems
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Figure 10-3 shows how a disk surface is divided into sectors and tracks. A 360K floppy disk is divided into 9 sectors per track and 40 tracks per side. Each sector is capable of holding 512 bytes. Simple math tells us the 512 bytes per sector times 9 sectors per track times 40 tracks per side times 2 sides equals 368,640 bytes. Cylinder Addressing Disk drives generally use the cylinder addressing method to store and retrieve data. In a disk drive, the read/write heads are positioned concurrently by parallel access arms to the same track number. In other words, if one head seeks track 20, then all heads move to track 20 of their respective recording surface. This means that all identically numbered tracks on the disk pack recording surfaces form a vertical cylinder. The cylinder number corresponds to the track number. All track 00s form cylinder 00. All track 200s form cylinder 200 and so on. Figure 10-4 shows an example of a disk drive seeking cylinder 20 of a disk pack. If a disk pack has 10 recording surfaces with 800 tracks per surface, then it would have 800 cylinders. Data is stored or retrieved by using the cylinder address. The cylinder address consists of the cylinder number, sector number, and head or recording surface number. Formatting As we have seen, formatting a disk writes the tracks and sectors on the disk. In addition, the format program used with personal computers also examines the disk for bad areas and creates the root directory, the file allocation table (FAT), and the disk boot sector. The boot sector contains information to tell the computer what type of disk is being used, what format the data is in, and other information that the DOS needs to read the disk. Fixed disks used in personal computers need an additional high-level format that defines the type of drive and the operating system being used. Once a disk is formatted, it is ready to have data written on it. How the data is stored in the sectors is primarily driven by the disk operating system (DOS). The following section shows how DOS organizes data on floppy and fixed disks by using directories. Directories DOS stores data in directories. A directory is a file system that enables DOS to manage files. There are two types of directories: the root directory and the subdirectories. Figure 10-3—Sectors and tracks on a magnetic disk. 10-4

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