Operating System Fundamental

An operating system is a program that acts as an intermediary between a user of 

a computer and  the computer hardware. The purpose of an operating system  is 

to provide an environment in which a user can execute program. 

An  operating  system  is  an  important  part  of  almost  every  computer  system. A 

computer system can be divided roughly into four components: the hardware, the 

operating system, the applications programs, and the users) 

The hardware ñ the central processing   unit (CPU), the memory, and the input / 

output  (I/O) devices  - provides  the basic computing  resources. The applications 

programs  ñ  such  as  compilers,  database  systems,  games,  and  business 

programs  ñ  define  the  ways  in  which  these  resources  are  used  to  solve  the 

computing problems of the users.  

An  operating  system  is  a  control  program.  A  control  program  controls  the 

execution of user programs to prevent errors and improper use of the computer. 

It is especially concerned with the operation and control of I/O devices. 

A  more  common  definition  is  that  the  operating  system  is  the  one  program 

running at all times on the computer (usually called the kernel), with all else being 

applications programs. 

Simple Batch Systems 

To speed up processing, jobs with similar needs were batched together and were 

run  through  the computer as a group. Thus,  the programmers would  leave  their 

programs with the operator. The operator would sort programs into batches with 

similar  requirements  and,  as  the  computer  became  available,  would  run  each 

batch.  The  output  from  each  job  would  be  sent  back  to  the  appropriate 

programmer

The  definitive  feature  of  a  batch  system  is  the  lack  of  interaction  between  the 

user and  the  job while  that  job  is executing. The  job  is prepared and submitted, 

and at some  later  time,  the output appears. The delay between  job submission 

and  job  completion  (called  turnaround  time)  may  result  from  the  amount  of 

computing needed, or from delays before the operating system starts to process 

the job. 

In  this  execution  environment,  the  CPU  is  often  idle.  This  idleness  occurs 

because  the speeds of  the mechanical  I/O devices are  intrinsically slower  than 

those of electronic devices). 

SPOOLING 

When  a  job  is  executed,  the  operating  system  satisfies  its  requests  for  card-

reader  input by  reading  from  the disk and when  the  job  requests  the printer  to 

output  a  line  that  line  is  copied  into  a  system  buffer and  is written  to  the disk. 

When the job is completed, the output is actually printed. This form of processing 

is called spooling  i.e. simultaneous peripheral operation on-line. Spooling, uses 

the disk as a huge buffer, for reading as far ahead as possible on input devices 

and for storing output files until the output devices are able to accept them. 

Spooling  is also used  for processing data at  remote sites. The CPU sends  the 

data  via  communications  paths  to  a  remote  printer.  The  remote  processing  is 

done  at  its  own  speed,  with  no  CPU  intervention.  The  CPU  just  needs  to  be 

notified when the processing is completed, so that it can spool the next batch of 

data. 

Spooling overlaps the I/O of one job with the computation of other jobs. Spooling 

has  a  direct  beneficial  effect  on  the  performance  of  the  system.  Spooling  can 

keep both the CPU and the I/O devices working at much higher rates 

 Multi-programmed Batched Systems 

Spooling provides an important data structure: a job pool. Spooling will generally 

result in several jobs that have already been read waiting on disk, ready to run. A 

pool of  jobs on disk allows  the operating system  to select which  job  to run next, 

to  increase  CPU  utilization. When  jobs  come  in  directly  on  cards  or  even  on 

magnetic tape, it is not possible to run jobs in a different order. Jobs must be run 

sequentially, on a first-come, first-served basis. However, when several jobs are 

on a direct-access device, such as a disk, job scheduling becomes possible.  

The most  important aspect of  job scheduling  is  the ability  to multiprogram. Off-

line operation and spooling for overlapped I/O have their limitations. A single user 

cannot,  in  general,  keep  either  the  CPU  or  the  I/O  devices  busy  at  all  times. 

Multiprogramming  increases  CPU  utilization  by  organizing  jobs  such  that  the 

CPU always has one to execute. 

Time-Sharing Systems 

Multiprogrammed  batched  systems  provide  an  environment  where  the  various 

system  resources  (for  example, CPU, memory,  peripheral  devices)  are  utilized 

effectively. 

Time sharing, or multitasking, is a logical extension of multiprogramming. Multiple 

jobs are executed by  the CPU switching between  them, but  the switches occur 

so frequently that the users may interact with each program while it is running.  

An  interactive,  or  hands-on,  computer  system  provides  on-line  communication 

between  the user and  the  system. The user gives  instructions  to  the operating 

system or to a program directly, and receives an immediate response. Usually, a 

keyboard  is used  to provide  input, and a display screen  (such as a cathode-ray 

tube (CRT) or monitor) is used to provide output. 

A  time-shared  operating  system  allows  the many  users  to  share  the  computer 

simultaneously. Since each action or command in a time-shared system tends to 

be short, only a little CPU time is needed for each user. As the system switches 

rapidly from one user to the next, each user is given the impression that she has 

her own computer, whereas actually one computer is being shared among many 

users. 

Time-sharing  operating  systems  are  even  more  complex  than  are 

multiprogrammed operating systems. As in multiprogramming, several jobs must 

be kept simultaneously  in memory, which  requires some  form of memory man-

agement and protection.  

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