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OPERATING SYSTEM INTRODUCTION
What is an Operating System?
Softwarethat controls the operation of a computer, directs the input and output of data,
keeps track of files, and controls the processing of computer programs.
Roles of Operating System
Its roles include managing the functioning of the computer hardware, running the applications
programs, serving as an interface between the computer and the user, and allocating
computer resources to various functions.
Services providedby an Operating System
 In a multitasking operating systemwhere multiple programs can be running at the same
time, the operating systemdetermines which applications should run in whatorder and
how much time should be allowed for each application before giving another application a
turn.
 Itmanages the sharing of internal memory among multiple applications.
 Ithandles input and output to and fromattached hardwaredevices, such as hard disks,
printers, and dial-up ports.
 Itsends messages to each application or interactive user (or to a systemoperator) about
the status of operation and any errors thatmay haveoccurred.
 Itcan offload the management of what are called batch jobs (for example, printing) so that
the initiating application is freed fromthis work.
 On computers that can provideparallel processing, an operating systemcan managehow
to divide the programso that it runs on more than one processor ata time.
Simple BatchSystem
 To speed up processing, jobs with similar needs werebatched together and were run
through the computer as a group.
 Thus, the programmers would leave their programs with the operator.
 The operator would sortprograms into batches with similar requirements and, as the
computer, became available, would run each batch.
 The output fromeach job would be sent back to the appropriateprogrammer
MultiprogrammedBatchSystem
 Multiprogrammed batch OS is that systemin which severaljobs are loaded in memory in
formof batch.
 The operating systempicks and begins to execute one of the jobs in the memory.
 Eventually the job may have to wait for sometask like an input output operation to be
complete
 In a multiprogrammed OS the OS is not idle it's simply switch to another job and executes it
.As there is always somejobs to execute, the CPU will never be idle.

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Advantages (MultiprogrammedBatchSystem)
 Utility of CPU get increased
 Better memory management
Disadvantages (Multiprogrammed Batch System)
 Output of a single job cannot be obtained until whole batch is not executed.
Time Sharing System
 Refers to the concurrent use of a computer by more than one user -- users share the
computer's time. Time sharing is synonymous with multi-user. Almost all mainframes and
minicomputers are time-sharing systems, but most personal computers and workstations
are not.
Parallel Systems
 Parallel computing is the use of two or more processors (cores, computers) in combination
to solve a single problem.
Advantage
 Increases the throughput.
Distributed Systems
 A distributed computer system consists of multiple software components that are on
multiple computers, but run as a single system.
 The computers that are in a distributed system can be physically close together and
connected by a local network, or they can be geographically distant and connected by a
wide area network.
 A distributed system can consist of any number of possible configurations, such as
mainframes, personal computers, workstations, minicomputers, and so on
 The goal of distributed computing is to make such a network work as a single computer.
Advantages of Distibuted Systems
 ResourceSharing
 Computational Speedup
 Reliability
 Communication
Real-Time Systems
 A systemin which the computer is required to performits tasks within the time controls of
some process or simultaneously with the systemit is assisting.
 Meet deadlines, it is required that the application has to finish certain tasks within the time
boundaries it has to respect.

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MainMemory
 Main memory is volatile which means that it is lost when the systemis shut off.
 Main memory & CPU registers are the only storage that CPU can access directly.
 The database does not usually fit in main memory, which is limited in size and must hold not only data but
systems programs and applications programs.
 However, any data that is to be processed must first be brought into a portion of main memory called the
buffer.
 When a record is requested, the system checks to see if it is already in buffer. If so, the systemsimply
passes the address in the buffer to the requesting process. If not, the systemfinds the location on disk
where the record is stored and directs the reading of the record into the buffer.
 Since accessing records in the buffer is many times faster than accessing those on disk, records that are
used most frequently are sometimes kept in the buffer whenever the database is in use.
 Main memory is erasable i.e. its lost it data if there is some kind of abnormal termination like system shut
off, application terminates etc
Magnetic Disks
Magnetic Tapes
 Magnetic tapes were used as a secondary storage medium.
 It can hold large quantities of data
 Random access to magnetic tapes is about a thousand times slower than random access to magnetic disks.
So tapes are not useful for secondary storage.
 A tape is kept in a spool, & is wound or rewound past a read-write head.
 Moving to the correct spot on a tape can take minutes, but once positioned, tape drives can write data at
speeds compared to disk drives.