Intel 80287, 80286 manual Support for Operating Systems, Organization of this Book

INTRODUCTION TO THE 80286

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Figure 1-1. Four Privilege Levels

1.3.4 Support for Operating Systems

Most operating systems involve some degree of concurrency, with multiple tasks vying for system resources. The task management mechanisms described above provide the 80286 with inherent support for such multi-tasking systems. Moreover, the advanced memory management features of the 80286 allow the implementation of sophisticated virtual memory systems.

Operating system implementors have found that a multi-level approach to system services provides better security and more reliable systems. For example, a very secure kernel might implement critical functions such as task scheduling and resource aiiocation, while less fundamenlal [ulictions (such as I/O) are built around the kernel. This layered approach also makes program development and enhancement simpler and facilitates error detection and debugging. The 80286 supports the layered approach through its four-level privilege scheme.

1.4 ORGANIZATION OF THIS BOOK

To facilitate the use of this book both as an introduction to the 80286 architecture and as a reference guide, the remaining chapters are divided into three major parts.

Part I, comprising chapters 2 through 4, should be read by all those who wish to acquire a basic familiarity with the 80286 architecture. These chapters provide detailed information on memory segmentation, registers, addressing modes and the general (application level) 80286 instruction set. In conjunction with the 80286 Assembly Language Reference Manual, these chapters provide sufficient information for an assembly language programmer to design and write application programs.

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