Evolution of operating system!!

1. Simple batch system
2. Multiprogrammed batch system
3. Time Sharing system
4. Personal Computer system
5. Parallel system
6. Distributed system
7. Real Time System

1.  Simple batch system:

In early ages of computers operating system was fairly simple. Its main task was to transfer control automatically from one job to the next.

                                   To speed up processing jobs with similar needs were batched together and run through the computer as a group. Thus, the programmers would leave their programs with the operators. The operators would sort programs into batches with similar requirements and run each batch.

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

A batch operating system normally reads a stream of separate job. It each with its own control cards that predefines what the job does. When the job is completed its output usually printed on line printer.

The main feature of a batch system is the lack of interaction between the user and the job while the job is executing.

Note

 The delay between the job submission and job completion called turn around time. In this execution environment the CPU is often ideal because of speed of mechanical i/o devices are slower than electronic devices.

Even a slow CPU in the range of micro seconds can executes 1000 instructions per seconds and a fast card reader on the other hand might reads 1200 cards/minutes.

To improve its performance we use the technique spooling.

When a job is executed, the operating system is satisfies its request for card reader input by reading from the disk. Similarly when the job request 

2. Multiprogrammed Batch Systems

 

Spooling provides an important data structure as a job pool.

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 job in a different order. Jobs must be run sequentially or FCFS method.

In a multiprogramming system, the OS simply switches to and executes another jobs. When that job needs to wait, the CPU switched to another job and so on.

As long as there is always some jobs to execute and the CPU will never be ideal.

Advantages of Multiprammed Batch System

1. Waiting time is very less.
2. Efficient use of memory and I/O devices.
3. Increased CPU utilization.
4. Higher throughput.

3. Time-Sharing System

Multiprogrammed Batch System provides an environment where the various system resources as CPU, Memory and I/O devices are utilized efficiently.

 Difficulties with the Batch System from the User Point of View

User cannot interact with the job when it is executing, the user must setup the control cards to handle all possible outcomes.

In multiple jobs, subsequent steps may depend on the result of: -

1. The running of a program may depend on successful completion. It can be difficult to define completely what to do in all cases.
2. Another difficulty, that programmer cannot modify a program as it executes.

Time Sharing or Multi Tasking is logical extension of multiprogramming. Multiple jobs are executed by the CPU switching between them, but the switches occurs so frequently that the users may interact with each program while it is running.

Time sharing systems were developed to provide interactive use of a computer system at a reasonable cost. A time shared OS uses CPU scheduling and multiprogramming to provide each user with a small portion of a memory.

A program that is loaded into memory and is executing is referred as a process.