Layers of an Operating System

Most Operating Systems are designed as a series of layers, expanding outward from the hardware to the application programs. This is particularly true of multiprogramming / multiuser systems which need to be concerned about security and protection of individual tasks from other tasks running concurrently in the same system.

The Major Layers

Access Control

Access control is typically achieved by reserving a collection of bits within the general "Flag Register" to indicate the current "access control level". Certain instructions are restricted so that they are only legal if the "access control level" is at least some minimum value; otherwise the instruction generates an "illegal operation" interrupt.

Last Updated on 16 May 2003 by Alan T. Pinck