سوال ۲۰ کنکور ارشد علوم کامپیوتر ۱۴۰۱

PASSAGE 1

There are two kinds of secondary memory technologies: sequential and random-access or direct-access. An example of the former is a strip of plastic, usually half an inch wide, coated on one side with metal oxide that can be magnetized. Tapes come in varying lengths of 250, 600, 1200, and 2400 feet and are usually kept off-line in libraries. When the information stored on tape is needed, an operator mounts the tape onto a tape drive which has a fixed reel and an empty hub. The tape is then threaded through the machine in a way similar to that of threading a tape through a tape recorder.

To mark the beginning and the end of a tape, small pieces of silver foil or any other reflective strips called tape-marks are stuck onto the tape. Information is then stored on the tape in magnetized units called bits which are similar to the bits in internal memory. Vertically, they form patterns such that every nine of them (eight bits of data plus a parity bit, used for error detection) are called a frame. A group of these frames forms a record of information, which may be either long or short. These records are separated from each other by marks called interrecord gaps, and a group of such records forms a block. Reading data from a 6250 BPI density tape is like reading about 15,000 cards in one second! Tapes are obviously a faster medium than punched cards for accessing information; moreover, they are a convenient way to transfer data from one computer to another, or even from one city to another.

20- ---------- to mark the beginning and the end of a tape.