Although two or more user programs reside in the main

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Unformatted text preview: orm numerical calculations, with little I/O operations. They are so called because they heavily utilize the CPU during the course of their processing. Programs used for scientific and engineering computations usually fall in this category of jobs. 2. I/O-bound jobs. These jobs normally input vast amount of data, perform very little computation, and output large amount of information. They are so called because during the course of their processing, their CPU utilization is very low and most of the time they perform I/O operations. Programs used for commercial data processing applications usually fall in this category of jobs. In a uniprogramming system, the CPU will be idle whenever the job that is currently being processed by the system performs I/O operations. With CPUbound jobs, the CPU idle time may not be significant, but for I/O-bound jobs the CPU may be idle 80 to 90% of the time. Moreover, since I/O devices are slower than the CPU by 20 to 100 times, the CPU idle time is significant even for CPUbound jobs that perform little I/O. The concept of multiprogramming was introduced to minimize the idle time of the CPU by organizing multiple jobs in the system so that the CPU always has something to execute. How this is done is explained below. Multiprogramming is the name given to the interleaved execution of two or more different and independent programs by the same computer. In Figure 14.3 we have been introduced to the notion of having two programs in the main memory at the same time - the operating system for overall system control and the user program for performing user's job. In multiprogramming, this concept is carried one step further by placing two or more user programs in the main memory and executing them concurrently. With multiple user programs simultaneously residing in the main memory, when a user program that was executing (using the CPU) starts performing I/O operations, the CPU is allocated to another user program in the main memory that is ready to use the CPU instead of allowing the CPU to be i...
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