Time-sharing vs. Multiprocessing

Difference Between Time-sharing And Multiprocessing

Time-sharing systems enable many terminal operators to use a computer at the same time. A basic terminal for timesharing consists of a keyboard, a monitor, and a printer. In addition to these essential components, an “intelligent” terminal has its own microprocessor and can function as a small computer when it is not being used as a terminal. Terminals are connected—often by phone lines or cable—to a centrally located computer, which is usually a large mainframe or shared server.

In a time-sharing system a large number of users sit at their own terminals online to the central computer. Each user is working on a different task. One may be writing a program; a second debugging a program; a third sending data to a program; a fourth reading output; and so on. Each user appears to have sole control of the computer, as if each were the only user. Actually, however, each user is sharing the machine with many—perhaps thousands—of other users.

The key to time-sharing is the fast processing capability of the computer relative to the speed with which people can use a terminal device. Most of the time, each terminal is busy sending or receiving data to or from the computer’s memory banks. During this time the computer can be processing data or solving problems for many other terminals. For example, after a terminal has transmitted a request for information to the computer’s memory, the computer may need only a fraction of a second to take the request from memory, perform the actual processing needed to answer the request, and store the result in memory for return transmission. During the time needed for the result to be transmitted back to the terminal and for the terminal operator to respond with a new query, the computer can be processing data for hundreds of other terminals. But the user of each terminal seems to have the computer’s full attention.

A familiar example of time-sharing is provided by flight-reservation systems for air travel. A system may have thousands of terminals in offices of airlines and travel agents across the country. These terminals are tied to a central computer facility through an extensive set of communication links. A request from a travel agent for space on a certain flight is input directly to the central computer, where data on all flights are stored in a large hard-disk memory bank. After the request has been keyed in, the CPU of the computer system processes the request using the flight information stored on the disk. The output, such as a flight confirmation, is then transmitted back to the agent. If the agent’s client buys a ticket, the ticketing information is transmitted to the computer, which stores it for future use and subtracts one seat from the space available.

Just as time-sharing systems let one computer work nearly simultaneously at many jobs, multiprocessing systems have two or more CPUs assigned to a single function. Multiprocessing systems are used for applications where the breakdown of a single main computer cannot be tolerated, including flight-reservation systems and the strategic defense systems used by the military. Often time-sharing and multiprocessing are combined in the same system, as in the case of flight-reservation systems.

 

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