Wednesday, March 4, 2009

Mainframe Systems

1.2 Mainframe Systems

Mainframe computer systems were the first computer used for many commercial and scientific applications. The operating system in mainframe computer used three concept.

1. Batch Systems
2. Multiprogramming Systems
3. Timeshared Systems

1. Batch Systems :

In batch systems, the user prepared a job which consisted of the program and the data. The job was usually stored in punch cards which was an input device for a mainframe systems. The output consisted of the result of the program.

The task of operating systems in mainframe computer was very simple. It loads the job [batched together job] into the main memory. When it finished, it loads the next job. Therefore, only one process was loaded into memory at a time.

Drawback of Mainframe Systems :

In batch system, to read the program, we need of a card reader. It was very slow in reading. The speed of the CPU was high when compared to speed the card reader. Therefore, most of the time, the CPU was idle.

Multiprogrammed Systems :

Multiprogramming systems overcomes the drawback of batch system. The basic idea of multiprogramming system is as follows :

The operating system keeps multiple jobs in main memory.

The operating system picks one of the jobs from the main memory and executes. If the running job needs of some input from the keyboard, it goes to waiting state. At that moment, some other job is picked up from main memory by operating system and that job is executed by CPU. Therefore, the CPU will never become idle.

Time-Sharing Systems : (CPU Scheduling + Multiprogramming)

Time sharing system is the extension of multiprogramming systems. In time sharing systems, the CPU switches so frequently from one job to another, so that each and every job gets the CPU time. Thus, the user can interact with each program while it is running.

A time-shared operating system allows 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 the entire computer system is dedicated to the user, even though it is shared by many users.

In time-shared systems, several jobs must be kept simultaneously in main memory, so the operating system must have

1. Memory Management
2. Protection

Time-sharing is achieved through virtual memory techniques. The main advantage of the virtual memory scheme is that programs can be larger than physical memory.

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