Operating system is an soft are that manages the computer hardware as well as providing an enviorment in which application programs can run.The most visible aspect of operating system is the interface to the computer system it provides to the human user.

Operating system provide a number of services.At lowest level,system calls allow a running program to make requests from the operating system directly.At a higher level,the command interpreter or shell porvides a mechnism for a user to issue a request without writing a program.The types of request vary according to level.

System services can be classified into several categories:program control,status request,and I/O requests.Program errors can be considered implicit requests for service.

Once the system services are defined,the struvture of the operating system can be deceloped.Various tables are needed to record the information that defines the state of the computer system and the status of the system\'s jobs.

The viural-machine concept takes the layered approach and treats both the kernel of the operating system and the hardware as though they were hardware.

OS are now almost written in a systems-implemention language or in a higher-level language.This feature improves their implementation,maintenance and portability.