Functions of OS
As you
know, operating system is responsible for functioning of the computer system.
To do that it carries out these three broad categories of activities −
· Essential functions − Ensures optimum and effective utilization of
resources
· Monitoring functions − Monitors and collects information related to
system performance
· Service functions − Provides services to users
Let us
look at some of the most important functions associated with these activities.
Processor management
Managing
a computer’s CPU to ensure its optimum utilization is called processor
management. Managing processor basically involves allocating processor time to
the tasks that need to be completed. This is called job scheduling. Jobs
must be scheduled in such a way that −
- There is
maximum utilization of CPU
- Turnaround
time, i.e. time required to complete each job, is minimum
- Waiting time is
minimum
- Each job gets
the fastest possible response time
- Maximum
throughput is achieved, where throughput is the average time taken to
complete each task
There
are two methods of job scheduling done by operating systems −
- Preemptive
scheduling
- Non-Preemptive
scheduling
Preemptive Scheduling
In
this type of scheduling, next job to be done by the processor can be scheduled
before the current job completes. If a job of higher priority comes up, the
processor can be forced to release the current job and take up the next job.
There are two scheduling techniques that use pre-emptive scheduling −
· Round robin scheduling − A small unit of time called time slice is
defined and each program gets only one time slice at a time. If it is not
completed during that time, it must join the job queue at the end and wait till
all programs have got one time slice. The advantage here is that all programs
get equal opportunity. The downside is that if a program completes execution
before the time slice is over, CPU is idle for the rest of the duration.
· Response ratio scheduling − Response ratio is defined as
ElapsedTimeExecutiontimereceivedElapsedTimeExecutiontimereceived
A job
with shorter response time gets higher priority. So a larger program may have
to wait even if it was requested earlier than the shorter program. This
improves throughput of the CPU.
Non-preemptive Scheduling
In
this type of scheduling, job scheduling decisions are taken only after the
current job completes. A job is never interrupted to give precedence to higher
priority jobs. Scheduling techniques that use non-preemptive scheduling are −
· First come first serve
scheduling − This is the simplest
technique where the first program to throw up a request is completed first.
· Shortest job next
scheduling − Here the job that needs
least amount of time for execution is scheduled next.
· Deadline scheduling − The job with the earliest deadline is
scheduled for execution next.
Memory Management
Process
of regulating computer memory and using optimization techniques to enhance
overall system performance is called memory management. Memory space is
very important in modern computing environment, so memory management is an
important role of operating systems.
As you
know, computers have two types of memory – primary and secondary.
Primary memory is fast but expensive and secondary memory is cheap
but slower. OS has to strike a balance between the two to ensure that system
performance is not hurt due to very less primary memory or system costs do not
shoot up due to too much primary memory.
Input
and output data, user instructions and data interim to program execution need
to be stored, accessed and retrieved efficiently for high system performance.
Once a program request is accepted, OS allocates it primary and secondary
storage areas as per requirement. Once execution is completed, the memory space
allocated to it is freed. OS uses many storage management techniques to keep a
track of all storage spaces that are allocated or free.
Contiguous Storage Allocation
This
is the simplest storage space allocation technique where contiguous memory
locations are assigned to each program. OS has to estimate the amount of memory
required for the complete process before allocation.
Non-contiguous Storage Allocation
As the
name suggests, program and associated data need not be stored in contiguous
locations. The program is divided into smaller components and each component is
stored in a separate location. A table keeps a record of where each component
of the program is stored. When the processor needs to access any component, OS
provides access using this allocation table.
In a
real-life scenario primary memory space might not be sufficient to store the
whole program. In that case, OS takes the help of Virtual Storage technique,
where program is physically stored in secondary memory but appears to be stored
in primary memory. This introduces a miniscule time lag in accessing the
program components. There are two approaches to virtual storages −
· Program paging − A program is broken down into fixed
size page and stored in the secondary memory. The pages are
given logical address or virtual address from 0 to n. A page
table maps the logical addresses to the physical addresses, which is used
to retrieve the pages when required.
· Program segmentation − A program is broken down into logical units
called segments, assigned logical address from 0 to n and stored in
secondary memory. A segment table is used to load segments from
secondary memory to primary memory.
Operating
systems typically use a combination of page and program segmentation to
optimize memory usage. A large program segment may be broken into pages or more
than one small segments may be stored as a single page.
File Management
Data
and information is stored on computers in form of files. Managing file system
to enable users to keep their data safely and correctly is an important
function of operating systems. Managing file systems by OS is called file
management. File management is required to provide tools for these file related
activities −
- Creating new
files for storing data
- Updating
- Sharing
- Securing data
through passwords and encryption
- Recovery in
case of system failure
Device Management
The
process of implementation, operation and maintenance of a device by operating
system is called device management. Operating system uses a utility software
called device driver as interface to the device.
When
many processes access the devices or request access to the devices, the OS
manages the devices in a way that efficiently shares the devices among all
processes. Processes access devices through system call interface, a
programming interface provided by the OS.
