It is a program with following features: An operating system is a program that acts as an interface between the software and the computer hardware. It is an integrated set of specialised programs that are used to manage overall resources and operations of the computer. It is specialised software that controls and monitors the execution of all other programs that reside in the computer, including application programs and other system software. Objectives of Operating System To make a computer system convenient to use in an efficient manner To hide the details of the hardware resources from the users To provide users a convenient interface to use the computer system To act as an intermediary between the hardware and its users and making it easier for the users to access and use other resources To manage the resources of a computer system To keep track of who is using which resource, granting resource requests, according for resource using and mediating conflicting requests from different programs and users To provide efficient and fair sharing of resources among users and programs Characteristics of Operating System Memory Management -- keeps tracks of primary memory i.e. what part of it is in use by whom, what part is not in use etc. and allocates the memory when a process or program requests it. Processor Management -- allocates the processor(CPU) to a process and deallocates processor when it is no longer required. Device Management -- keeps track of all devices. This is also called I/O controller that decides which process gets the device, when, and for how much time. File Management -- allocates and de-allocates the resources and decides who gets the resources. Security -- prevents unauthorized access to programs and data by means of passwords and similar other techniques. Job accounting -- keeps track of time and resources used by various jobs and/or users. Control over system performance -- records delays between request for a service and from the system. Interaction with the operators -- The interaction may take place via the console of the computer in the form of instructions. Operating System acknowledges the same, does the corresponding action and informs the operation by a display screen. Error-detecting aids -- Production of dumps, traces, error messages and other debugging and error-detecting methods. Coordination between other software and users -- Coordination and assignment of compilers, interpreters, assemblers and other software to the various users of the computer systems.