An Operating Systems Vade Mecum

Device hardware 149Disks are the most common devices for backing and secondary store. They come in awide variety of capacities, speeds, and prices, but all disks share some common characteristics.Disks accept and provide data at a high rate (approximately 2 megabits persecond). Data are transferred to and from the disk in increments of one block, which istypically between 256 and 1024 bytes. It is generally impossible to read or write lessthan one block from or to a disk. If the operating system wishes to write out only a fewbytes without destroying the rest of the block, it must first read in the block, modify thosefew bytes, and then write out the entire block.The disk unit contains a disk pack, which is sometimes designed to be removable.The disk pack physically resembles a stack of phonograph records, called platters.Information is recorded magnetically, usually on both sides. All the platters rotate as oneunit, usually at 60 rotations per second. Each platter is divided into bands called tracks.There are typically on the order of 200 tracks per side on a disk pack, but some disks,like the IBM 3380, have almost 900, and some newer Winchester disks, which have asealed head-disk assembly, have more than 1500. Unlike phonograph record grooves,the tracks do not spiral toward the center but are concentric. Each track is divided intosectors, each of which holds one block of information. There are about 30 sectors in atrack on a large disk pack. Figure 5.1 shows the layout of a disk.Floppy disk drives are similar, but they accommodate only one platter, which canbe removed. Some only use one side of the platter. Floppies come in two standard sizes(8 inches and 5 1 ⁄4 inches) and various densities. They rotate more slowly than conventionaldisks. If their motors are allowed to run continually, both the motors and thefloppies wear out very quickly, so the motors are only turned on when a transfer needs toread/writeheadsplattersFigure 5.1 Structure of a disk packtop view