System Programs

System programs provide a convenient environment for program development and execution.

They can be divided into:

   1. File manipulation

   2. Status information

   3. File modification

   4. Programming language support

   5. Program loading and execution

   6. Communications

   7. Application programs

Most users’ view of the operation system is defined by system programs, not the actual system calls.

MS-DOS System Structure

MS-DOS – written to provide the most functionality in the least space

   1. not divided into modules

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

MS-DOS Layer Structure

UNIX System Structure

UNIX – limited by hardware functionality, the original

UNIX operating system had limited structuring. The UNIX

OS consists of two separable parts.

   1. Systems programs

   2. The kernel

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

   4. Provides the file system, CPU scheduling, memory management, and other operating-system functions;

       a large number of functions for one level.

UNIX System Structure

Silberschatz, Galvin and Gagne 2002 3.25 Operating System Concepts

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.

An Operating System Layer

Microkernel System Structure

Moves as much from the kernel into “user” space.

Communication takes place between user modules using message passing.

Benefits:

easier to extend a microkernel

easier to port the operating system to new architectures more reliable (less code is running in kernel mode)

more secure

Windows NT Client-Server Structure