METHODS OF OPERATIONS AND MODES OF ACCESS

Operating systems vary considerably in their capabilities, from relatively simple user microcomputer system, to sophisticated mainframe computer. OS are often described in terms of the methods by which they operate the system and the modes of system access they provide. Some of them are introduced here:

1) Single program operation: The OS supervises the loading and running of one program at a time and the input and output of data from and to peripheral devices.

2) Multiprocessing: In larger systems, one computer system may have more than one processor, but may still share all or some of the same memory. In such cases two programs may be processed at the same time instant (happening immediately).

3) Multiprogramming: Multiprogramming is the term used to describe the technique of having more than one program in the computer's memory at the same time. A CPU running a single program spends most of its time idle. waiting for input or output. Multiprogramming helps to ensure that the CPU runs in the most efficient way possible. The operating system has to allocate resources to each program, allowing each one a small amount of processor time before moving to the next one. There are many different types of multiprogramming systems:

A Multi-user system allows several people to be running programs simultaneously from different workstation, this is also called time-sharing system, with each user in turn being allocated a time-slice. 

Multi-Tasking on the other hand can carry many tasks at the samelime (i.e. they can run several programs simultaneously). The main advantage of this system is that when running interactive programs, the computer spends most of the time waiting for you to do something. 

Meanwhile it could be attending other tasks or making other computationsinstead of wasting time.

Batch Processing : Batch Processing is based on the idea of automatic job-to-job transaction facility provided by all operating systems. In batch mode, each user prepares his program off-line and submits it to the computer center. Batch processing is also known as serial, sequential, off-line or stacked job processing. When a computer is used in this way, the input data are introduced into the computer and processed automatically generally without operator’s invention.

The job is not processed until fully input. The jobs are entered and stored on a disk in a batch queue and then run one or more at a time under the control of the operating system. A job may wait for minutes or hours spending on a workload.

Advantage:

• User can enter the list of jobs at once and leave the computer.

• It reduces the idle time of a computer system because transition form one job to another doesn’t operator intervention.

Disadvantage:

• Jobs may take hours to compete and no interaction between task and user.
• Lots of memory required. 
• It is difficult to provide desired priority.

Remote job entry: - It refers the batch processing where jobs are entered at a terminal remote (away) from the computer and transmitted into the computer.
Multi-aeeess: - This occurs if the computer allows interactive facilities to more than one user at a time.

Time-sharing: - Many people working at the same time allowing many input-output devices to be connected so that many people can communicate with the computer at the same lime (each with his/her own terminal). In order to sort out the different tasks to carry on for each different user the computer gives each one a small slice of lime switching very fast between users. This is known as time-sharing. In the other word, processor time is divided into small units called time slices and shared in turn between users to provide multi-access.

Real-time processing: - Real time systems can be of different types: process control, information storage and retrieval, and transaction processing. In any of these systems, the data input to the computer must be processed immediately so that the results are available to influence the activity currently taking place. There is often a need for multiprocessing and a front-end processor in these systems.