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Virtual Memory
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Virtual memory
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Demand paging and demand segmentation mean
that, when a program is being executed, part of
the program is in memory and part is on disk.
This means that, for example, a memory size of
10 MB can execute 10 programs, each of size
3 MB, for a total of 30 MB.
At any moment, 10 MB of the 10 programs are in
memory and 20 MB are on disk.
There is therefore an actual memory size of
10 MB, but a virtual memory size of 30 MB
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Virtual memory (Cont.)
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Virtual memory, which implies demand paging,
demand segmentation or both, is used in almost all
operating systems today.
Figure below shows the concept.
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OS Components- Process manager
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A second function of an operating system is process management,
but before discussing this concept, we need to define some terms.
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A program is a non-active set of instructions stored on disk. It may
or may not become a job
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A program becomes a job from the moment it is selected for
execution until it has finished running and becomes a program
again.
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A process is a program in execution. It is a program that has started
but has not finished.
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State diagrams
The relationship between a program, a job and
a process becomes clearer if we consider how
a program becomes a job and how a job
becomes a process.
 This can be illustrated with a state diagram
that shows the different states of each of these
entities.
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State diagrams (Cont.)
Figure shows a State diagram with boundaries between program, job and process
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State diagrams (Cont.)
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A program becomes a job when selected by the operating system
and brought to the hold state
It remains in this state until it can be loaded into memory
When there is memory space available to load the program totally
or partially, the job moves to the ready state. It now becomes a
process
It remains in memory and in this state until the CPU can execute it,
moving it to the running state at this time
When in the running state, one of three things can happen:
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The process executes until it needs I/O resources, it goes into the waiting
state
The process exhausts its allocated time slot, it goes into the ready state
The process terminates, it goes into the terminated state
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Schedulers
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To move a job or process from one state to
another,
The process manager uses two schedulers
 The
job scheduler
 The process scheduler
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Schedulers (Cont.)
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Job scheduler:
Moves a job from the hold state to the ready state or
from the running state to the terminated state
(Figure shows job scheduler)
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Schedulers (Cont.)
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Process scheduler:
Moves a process between the running, waiting, and
ready states many times before it goes to the
terminated state and is no longer a process
As shown in the figure below
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Queuing
In reality, there are many jobs and many
processes competing with each other for
computer resources.
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Queuing
To handle multiple processes and jobs, the
process manager uses queues (waiting lists).
 A job control block or process control block is
associated with each job or process.
 This is a block of memory that stores
information about that job or process.
 The process manager stores the job or process
control block in the queues instead of the job
or process itself.
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Queuing (Cont.)
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Figure shows the circulation of jobs and processes
through three queues ( the job queue, the ready
queue , and the I/O queue)
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Queuing (Cont.)
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The job queue holds the jobs that are waiting for
memory
The ready queue holds the processes that are in
memory, ready to be run and waiting for the CPU
The I/O queue holds the processes that are waiting
for an I/O device (one queue for each I/O device)
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Selecting next job or process
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The process manager can have different policies for
selecting the next job or process from a queue:
It could be first in first out (FIFO), shortest length
first, highest priority first, and so on
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Process synchronization
The whole idea behind process management is
to synchronize different processes with
different resources.
 Whenever resources can be used by more than
one user (or process, in this case), we can have
two problematic situations: deadlock and
starvation.
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Deadlock on a narrow bridge
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Deadlock
Deadlock occurs when the operating system does not
put resource restrictions on processes.
 There are four necessary conditions for deadlock to
occur:
Mutual exclusion only one process can hold a resource
 Resource holding A process holds a resource even though it
cannot use it until other resources are available
 No preemption The OS cannot temporarily reallocate a
resource
 Circular waiting All processes and resources involved form a
loop
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Deadlock example
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Assume that there are two processes, A and B
Process A is holding File 1, and it has requested File
2 and cannot release File 1 until it acquires File 2
Process B is holding File 2, and it has requested File
1 and cannot release File 2 until it acquires File 1
Files in most system are not sharable
If there is no provision in this situation to force a
process to release a file, deadlock is created (as
shown in figure below)
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Deadlock example (Cont.)
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How the system preventing deadlock?
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One solution is not to allow a process to start running
until the required resources are free, but this creates
another problem (Starvation)
The second solution is: do not allow one of deadlock
conditions to happen (i.e. limit the time a process can
hold a resource)
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The dining philosophers problem
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Starvation
Starvation is the opposite of deadlock. It can happen
when the operating system puts too many resource
restrictions on a process.
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Starvation example
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For example, imagine an operating system that
specifies that a process must have possession of its
required resources before it can be run
In Figure below,
Imagine that process A needs two files, File1 and File2
 File1 is being used by process B and File2 is being used by
process E
 Process B terminates and releases File1
 Process C, which needs only File1, is allowed to run
 Process E terminates and releases File2
 Process D, which needs only File2, is allowed to run
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Starvation example (Cont.)
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OS Components- Device manager
The device manager, or input/output manager,
is responsible for access to input/ output
devices.
 There are limitations on the number and speed
of input/output devices in a computer system.
 The device manager is responsible for the
efficient use of input/output devices
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Device manager responsibilities
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The device manager monitors every input/output
device constantly to ensure that the device is
functioning properly.
The device manager maintains a queue for each
input/output device or one or more queues for
similar input/output devices.
The device manager controls the different policies
for accessing input/output devices. i.e. it may use
FIFO for one device and shortest length first for
another
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OS Components- File manager
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Operating systems today use a file manager to
control access to files. The responsibilities of a file
manager:
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Controls access to files. The type of access can vary
(read, write, execute)
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Supervises the creation, deletion, and modification of
files.
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Controls the naming of files.
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Supervises the storage of files. ( how, where,.. etc)
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is responsible for archiving and backups.
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Any Questions ?
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References
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Behrouz Forouzan and Firouz Mosharraf,
“Foundations of computer science”, Second edition,
chapter7, pp. 188-203
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