Time-sharing
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- Alternate uses: see Timesharing
Time-sharing is an approach to interactive computing in which a single computer is used to provide apparently simultaneous interactive general-purpose computing to multiple users by sharing processor time.
Because early mainframe computers were extremely expensive, it was not possible to allow a single user exclusive access to the machine for interactive use. But because computers in interactive use often spend much of their time idly waiting for user input, it was suggested that multiple users could share a machine by using one user's idle time to service other users. Similarly, small slices of time spent waiting for disk, tape, or network input could be granted to other users. Computers capable of providing time-sharing services would usually operate in batch mode overnight.
These solutions alone were not sufficient to build a fully functional time-sharing system. In order to provide smooth service to multiple users, a time-sharing system needed a way to deal with multiple processes that did not frequently pause for input/output. This required a hardware interrupt system capable of pausing a running process, and giving processor time to another process.
The concept was first described publicly in early 1957 by Bob Bemer as part of an article in Automatic Control Magazine. The first project to implement a time-sharing system was initiated by John McCarthy in late 1957. Although he left to work on Project MAC and other projects, one of the results of the project, known as the Computer Time Sharing System or CTSS, was demonstrated in November, 1961. CTSS has a good claim to be the first time-sharing system and remained in use until 1973. The first commercially successful time-sharing system, and the one which became the most widespread in the late 1960s and early 1970s, was the Dartmouth Time-Sharing System (DTSS) which was first implemented at Dartmouth College in 1964 and subsequently formed the basis of General Electric's computer bureau services.
Other early time-sharing systems — some still in widespread use — include:
- IBM CMS (part of VM/CMS)
- IBM TSS/360 (never finished; see OS/360)
- IBM Time Sharing Option (TSO)
- Michigan Terminal System
- Multics
- MUSIC/SP
- WYLBUR
Christopher Strachey is sometimes credited with the invention of the time-sharing concept. However, although he used the term time-sharing for it, the concept which he described is nowadays known as multitasking. Time-sharing is related to multitasking in that both systems involve a single computer processor executing multiple processes in an apparently simultaneous manner. Time-sharing, however, refers to a computer supporting multiple simultaneous users, while multitasking more broadly encompasses the simultaneous execution of multiple processes, regardless of the number of users.
External link
- DEC's PDP-6 was the worlds first commercial time-sharing system (http://americanhistory.si.edu/csr/comphist/bell.htm#first%20commercial%20timesharing)
de:Time-Sharing fr:Temps partagé it:Time-sharing ja:タイムシェアリングシステム