Page 178 - DCAP403_Operating System

Unit 9: I/O & Secondary Storage Structure

The drive head sweeps across the entire surface of the disk, visiting the outermost cylinders Notes
before changing direction and sweeping back to the innermost cylinders. It selects the next
waiting requests whose location it will reach on its path backwards and forwards across the disk.
Thus, the movement time should be less than FCFS but the policy is clearly fairer than SSTF.
Circular SCAN (C-SCAN)

C-SCAN is similar to SCAN but I/O requests are only satisfied when the drive head is traveling
in one direction across the surface of the disk. The head sweeps from the innermost cylinder to
the outermost cylinder satisfying the waiting requests in order of their locations. When it reaches
the outermost cylinder it sweeps back to the innermost cylinder without satisfying any requests
and then starts again.
Look

Similarly to SCAN, the drive sweeps across the surface of the disk, satisfying requests, in
alternating directions. However the drive now makes use of the information it has about the
locations requested by the waiting requests.

Example: A sweep out towards the outer edge of the disk will be reversed when there are
no waiting requests for locations beyond the current cylinder.

Circular LOOK (C-LOOK)

Based on C-SCAN, C-LOOK involves the drive head sweeping across the disk satisfying requests
in one direction only. As in LOOK the drive makes use of the location of waiting requests in order
to determine how far to continue a sweep, and where to commence the next sweep. Thus it may
curtail a sweep towards the outer edge when there are locations requested in cylinders beyond
the current position, and commence its next sweep at a cylinder which is not the innermost one,
if that is the most central one for which a sector is currently requested.

9.7 Disk Management

The hard disk is the secondary storage device that is used in the computer system. Usually the
primary memory is used for the booting up of the computer. But a hard disk drive is necessary
in the computer system since it needs to store the operating system that is used to store the
information of the devices and the management of the user data.

The management of the IO devices that is the Input Output devices, like the printer and the other
peripherals like the keyboard and the etc; all require the usage of the operating system. Hence the
information of the all such devices and the management of the system are done by the operating
system. The operating system works as an interpreter between the machine and the user.
The operating system is a must for the proper functioning of the computer. The computer is a
device that needs to be fed with the instructions that are to be carried out and executed. Hence
there needs to be an interpreter who is going to carry out the conversions from the high level
language of the user to the low level language of the computer machine.

Task If your system not proper shut down what happen when you restart the
system.

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