Wednesday, March 21, 2007


While a computer may be viewed as running one gigantic program stored in its main memory, in some systems it is necessary to give the appearance of running several programs simultaneously. This is achieved by having the computer switch rapidly between running each program in turn. One means by which this is done is with a special signal called an interrupt that reason the computer to stop executing instructions periodically where it was and do something else instead. By remembering where it was executing preceding to the interrupt, the computer may return to that task later on.

If several programs are running "at the same time", then the interrupt generator may cause several hundred interrupts per second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, lots of programs may seem to be running at the same time even though only one is ever executing at any given instant. This method of multitasking is sometimes termed "time-sharing" since each program is allocated a "slice" of time in turn.

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