System Structure

Simple Structure

MS-DOS – written to provide the most functionality in the least space
not divided into modules

Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated.

UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts.

• Systems programs

• The kernel

• Consists of everything below the system-call interface and above the physical hardware

• Provides the file system, CPU scheduling memory management, and other operating-system functions; a large number of functions for one level.

Layered Approach

The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers.