Page 249 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Problems: May cause starvation to some requests, since requests may arrive at anytime.
7.3.2 Scan

Head starts at one end and moves towards the other end, servicing the requests on its way. At
the end the head movement direction is reversed and servicing continues. Example, head position
at 53 movement towards zero, servicing 37, 14 and goes up to zero and then changes direction.

Figure 7.3: Scan

This algorithm is sometimes called “elevator” algorithm, since it resembles to the behaviour
of elevator.
7.3.3 Cscan

A variant of scan designed to provide a more uniform wait. Starts from one end and moves
towards the other end servicing all requests on its way. When the head reaches to the other
end, it immediately returns to the beginning of the disk, without servicing any request on its
return journey.

Figure 7.4: Cscan

Self Assessment

Multiple choice questions:
1. An online backing storage system capable of storing larger quantities of data is

(a) CPU (b) Memory
(c) Mass storage (d) Secondary storage
2. Which is an item of storage medium in the form of circular plate?
(a) Disk (b) CPU

(c) Printer (d) ALU

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