Department of Information Technology
Subject Code:CS3451
Subject Tittle: Itroduction To Operating System
Team Members: Reg No:
Maju mon 210621205031
Manoj M 210621205032
Manoj P 210621205033
Mariam Jerina S 210621205034
Niranchan P 210621205035

DISK STRUCTURE

INTRODUCTION:
• Disk Structure in Operating System: The actual physical details of a
modern hard disk may be quite complicated. Simply, there are one or
more surfaces, each of which contains several tracks, each of which is
divided into sectors. There is one read/write head for every surface of
the disk.

DEFFINITION:
• Modern disk drives are addressed as large one-
dimensional arrays of logical blocks, where the logical
block is the smallest unit of transfer. The size of a
logical block is usually 512 bytes, although some disks
can be low-level formatted to have a different logical
block size, such as 1,024 bytes. This option is described
in Section 12.5.1.

• The disk is dividedinto tracks.
• Each track is further dividedinto sectors.
• The point to be noted here is that outer tracks are bigger in size than the inner tracks but they containthe
same number of sectors and have equal storage capacity.
• This is because the storage density is high in sectors of the inner tracks whereas the bits are sparsely
arranged in sectors of the outer tracks.
• Some space of every sector is used for formatting.
• the actualcapacity of a sector is less than the given capacity.

• Read-Write(R-W) head moves over the rotatinghard disk.
• It is this Read-Write head that performs all the read and writes operationson the disk
• hence, the position of the R-W head is a major concern.
• To perform a read or write operationon a memory location,we need to place the R-W head over that
position.

• Seek time – The time taken by the R-W head to reach the desired
track from its current position.
• Rotational latency – Time is taken by the sector to come under the R-
W head.
• Data transfer time – Time is taken to transfer the required amount of
data. It depends upon the rotational speed.
• Controller time – The processing time taken by the controller.
• Average Access time – seek time + Average Rotational latency + data
transfer time + controller time.

• What Is The Function OF Disks?
• Magnetic disks can be used to store a wide range of data,
including operating systems, software, and other files. Hard
disk drives, in other words, can read and write data on hard
disks. There is an HDD for each computer, which can be used
as a primary or secondary storage device.

• Advantages –
1.First Come First Serve algorithm has a very simple logic, it executes
the process requests one by one in the sequence they arrive.
2.Thus, First Come First Serve is very simple and easy to understand
and implement.
3.In FCFS eventually, each and every process gets a chance to execute,
so no starvation occur.

• Disadvantages –
1.This scheduling algorithm is nonpreemptive, which means the
process can’t be stopped in middle of execution and will run it’s full
course.
2.The throughput of FCFS is not very efficient.
3.FCFS is implemented on small systems only where input-output
efficiency is not of utmost importance.