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Unformatted text preview: ecute. The operating system keeps several jobs in memory simultaneously,
as shown in Figure 2.2. This set of jobs is a subset of the jobs on the disk which are ready
to run but cannot be loaded into memory due to lack of space. Since the number of jobs
that can be kept simultaneously in memory is usually much smaller than the number of
jobs that can be in the job pool; the operating system picks and executes one of the jobs
in the memory. Eventually the job has to wait for some task such as an I/O operation to
complete. In a non multi-programmed system, the CPU would sit idle. In a multiprogrammed system, the operating system simply switches to, and executes another job.
When that job needs to wait, the CPU simply switches to another job and so on. Figure 2.2 Memory layout for a multi- programmed
2 Figure 2.3 illustrates the concept of multiprogramming by using an example system
with two processes, P1 and P2. The CPU is switched from P1 to P2 when P1 finishes its
CPU burst and needs to wait for an event, and vice versa when P2 finishes it CPU burst
and has to wait for an event. This means that when one process is using the CPU, the
other is waiting for an event (such as I/O to complete). This increases the utilization of
the CPU and I/O devices as well as throughput of the system. In our example below, P1
and P2 would finish their execution in 10 time units if no multiprogramming is used and
in six time units if multiprogramming is u...
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This document was uploaded on 10/17/2012.
- Spring '12