Secondary storage devices
Today and Yesterday I will describe some characteristics of magnetic disk and magnetic tape storage devices. Readers who have studied these devices already may just browse through this post.
From this post I will describe two topic....
1) Hardware Description of Disk Devices
2) Magnetic Tape storage devices
Hardware Description of Disk Devices
Magnetic disks are used for storing large amounts of data. The most basic unit of data on the disk is a single bit of information. By magnetizing an area on disk in certain ways, one can make it represent abit value of either 0 (zero) or 1 (one). To code information, bits are grouped into bytes (or characters). Byte sizes are typically 4 to 8 bits, depending on the computer and the device. We assume that one character is stored in a single byte, and we use the terms byte and character interchangeably. The capacity of a disk is the number of bytes it can store, which is usually very large. Small floppy disks used with microcomputers typically hold from 400 Kbytes to 1.5 Mbytes; hard disks for micros typically hold from several hundred Mbytes up to a few Gbytes; and large disk packs used with minicomputers and mainframes have capacities that range up to a few tens or hundreds of Gbytes. Disk capacities continue to grow as technology improves. A disk is single-sided if it stores information on only one of its surfaces and double-sided if both surfaces are used. To increase storage capacity, disks are assembled into a disk pack , which may include many disks and hence many surfaces. Information is stored on a disk surface in concentric circles of small width, each having a distinct diameter. Each circle is called a track. For disk packs, the tracks with the same diameter on the various surfaces are called a cylinder because of the shape they would form if connected in space. The concept of a cylinder is important because data stored on one cylinder can be retrieved much faster than if it were distributed among different cylinders.
The number of tracks on a disk ranges from a few hundred to a few thousand, and the capacity of each track typically ranges from tens of Kbytes to 150 Kbytes. Because a track usually contains a large amount of information, it is divided into smaller blocks or sectors. The division of a track into sectors is hard-coded on the disk surface and cannot be changed. One type of sector organization calls a portion of a track that subtends a fixed angle at the center as a sector . Several other sector organizations are possible, one of which is to have the sectors subtend smaller angles at the center as one moves away, thus maintaining a uniform density of recording . Not all disks have their tracks divided into sectors.
The division of a track into equal-sized disk blocks (or pages) is set by the operating system during disk formatting (or initialization). Block size is fixed during initialization and cannot be changed dynamically. Typical disk block sizes range from 512 to 4096 bytes. A disk with hard-coded sectors often has the sectors subdivided into blocks during initialization. Blocks are separated by fixed-size interblock gaps, which include specially coded control information written during disk initialization. This information is used to determine which block on the track follows each interblock gap.
In my next post I will represent specifications of a typical disk.
good post