Objectives and functions of Operating system

An OS is a program that controls the execution of application programs and acts as an interface between applications and the computer hardware. It can be thought of has having three objectives:

• Convenience: An OS makes a computer more convenient to use.
• Efficiency: An OS allows the computer system resources to be used in an efficient manner.
• Ability to evolve: An OS should be constructed in such a way as to permit the effective development, testing, and introduction of new system functions without interfering with service.

 The Operating System as a User/Computer Interface

Every general-purpose computer consists of the hardware, operating system, system programs, and application programs. The hardware consists of memory, CPU, ALU, I/O devices, peripheral devices, and storage devices. System program consists of compilers, loaders, editors, OS, etc. The application program consists of business programs, database programs.

Every computer must have an operating system to run other programs. The operating system coordinates the use of the hardware among the various system programs and application programs for various users. It simply provides an environment within which other programs can do useful work. 

Briefly, the OS typically provides services in the following areas:

Program development: The OS provides a variety of facilities and services, such as editors and debuggers, to assist the programmer in creating programs. Typically, these services are in the form of utility programs that, while not strictly part of the core of the OS, are supplied with the OS and are referred to as application program development tools.

Program execution: A number of steps need to be performed to execute a program. Instructions and data must be loaded into main memory, I/O devices and files must be initialized, and other resources must be prepared. The OS handles these scheduling duties for the user.

Access to I/O devices: Each I/O device requires its own peculiar set of instructions or control signals for operation. The OS provides a uniform interface that hides these details so that programmers can access such devices using simple reads and writes.

Controlled access to files: For file access, the OS must reflect a detailed understanding of not only the nature of the I/O device (disk drive, tape drive) but also the structure of the data contained in the files on the storage medium. In the case of a system with multiple users, the OS may provide protection mechanisms to control access to the files.

System access: For shared or public systems, the OS controls access to the system as a whole and to specific system resources. The access function must provide protection of resources and data from unauthorized users and must resolve conflicts for resource contention.

Error detection and response: A variety of errors can occur while a computer system is running. These include internal and external hardware errors, such as a memory error, or a device failure or malfunction; and various software errors, such as division by zero, attempt to access forbidden memory location, and inability of the OS to grant the request of an application. In each case, the OS must provide a response that clears the error condition with the least impact on running applications. The response may range from ending the pro- gram that caused the error, to retrying the operation, to simply reporting the error to the application.

Accounting: A good OS will collect usage statistics for various resources and monitor performance parameters such as response time. On any system, this information is useful in anticipating the need for future enhancements and in tuning the system to improve performance. On a multiuser system, the information can be used for billing purposes.

The Operating System as Resource Manager

A computer is a set of resources for the movement, storage, and processing of data and for the control of these functions. The OS is responsible for managing these resources.

User program and data is also stored in remaining parts of the memory. Allocation of main memory ts controlled by operating system with the help of memory management hardware.

I/O device is controlled by OS and it decides when an I/O device can be used by program in execution. processor is one type of resource and OS control the execution of user program on the processor.

Modern OS allows multiple programs to run at the same time. If multiple users are using computer then there is need of managing and protecting the memory I/O devices and other resources.

Resource management includes sharing resources in different ways. Time and space are the two concept for resource sharing.

Time : Time slot is allocated to each program first one gets to use the resource then another and so on.
Space : Consider the example of main memory. Main memory is normally divided up among several running programs, so each one can be resident at the same time.

Source: Operating System by William Stallings