An operating system is a program that manages the computer hardware. It also provides a basis for application programs and acts as a intermediary between computer user and computer hardware. The operating system is large and complex. It is divided into separate subsystems, each with carefully defined functions, input, output and function. This arrangement and relationship between subsystems or components of a complex system is called operating system structure. Simple Structure Some systems do not have a well defined structure. such system does not break the system into subsystems, and has no distinction between user and kernel modes. this system allows all programs direct access to the underlying hardware. MS DOS is the best example of such a system. A few people was originally designed and implemented it. And gradually DOS became popular and important. This simple layered approach is used by the original UNIX operating system. Almost all OS is in one large layer,
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