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Unformatted text preview: ibuted operating systems deal
with multiple computers interconnected together by a communication network. In
case of a network operating system the user views the system as a collection of
distinct computers, but in case of a distributed operating system the user views the
system as a "virtual uniprocessor".
There are several resources in a system for which the resource allocation policy
must ensure exclusive access by a process. For example, a printer needs to be
exclusively allocated to a process till the process finishes using it because its
simultaneous use by two or more processes will lead to garbled printed output.
Since a system consists of a finite number of units of each resource type (for
example, three printers, six tape drives, four disk drives, two CPUs, etc.), multiple
concurrent processes normally have to compete to use a resource. In this situation,
the sequence of events required to use a resource by a process is as follows:
The process first makes a request for the resource.
requested resource is not available, possibly because it is being used by another process, the requesting process must wait until the requested resource is allocated
to it by the system.
2. Allocate. The system allocates the resource to the requesting process as soon
as possible. It maintains a table in which it records whether each resource is free
or allocated and, if it is allocated, to which process. If the requested resource is
currently allocated to another process, the requesting process is "added to a queue
of processes waiting for this resource. Once the system allocates the resource to
the requesting process, that process can exclusively use the resource by operating
Release. After the process has finished using the allocated resource, it
releases the resource to the system. The system table records are updated at the
time of allocation and release to reflect the current status of availability of
The request and release of resources are system calls, such as request and release
for devices, open and close for files, and allocate and free for memory space.
Notice that of t...
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- Spring '14