Intel 80286, 80287 manual Chapter Protection

CHAPTER 7

PROTECTION

7.1 INTRODUCTION

In most microprocessor based products, the product's availability, quality, and reliability are deter- mined by the software it contains. Software is often the key to a product's success. Protection is a tool used to shorten software development time, and improve software quality and reliability.

Program testing is an important step in developing software. A system with protection will detect software errors more quickly and accurately than a system without protection. Eliminating errors via protection reduces the development time for a product.

Testing software is difficult. Many errors occur only under complex circumstances which are difficult to anticipate. The result is that products are shipped with undetected errors. When such errors occur, products appear unreliable. The impact of a software error is multiplied if it introduces errors in other bug-free programs. Thus, the total system reliability reduces to that of the least reliable program running at any given time.

Protection improves the reliability of an entire system by preventing software errors in one program from affecting other programs. Protection can keep the system running even when some user program attempts an invalid or prohibited operation.

Hardware protection performs run-time checks in parallel with the execution of the program. But, hardware protection has traditionally resulted in a design that is more expensive and slower than a system without protection. However, the 80286 provides hardware-enforced protection without the performance or cost penalties normally associated with protection.

The protected mode 80286 implements extensive protection by integrating these functions on-chip. The 80286 protection is more comprehensive and flexible than comparable solutions. It can locate and isolate a large number of program errors and prevent the propagation of such errors to other tasks or programs. The protection of the total system detects and isolates bugs both during development and installed usage. Chapter 9 discusses exceptions in more detail.

The remaining sections of this chapter explain the protection model implemented in the 80286.

7.1.1 Types of Protection

Protection in the 80286 has three basic aspects:

I.Isolation of system software from user applications.

2.Isolation of users from each other (Inter-task protection).

3.Data-type checking.

The 80286 provides a four-level, ringed-type, increasingly-privileged protection mechanism to isolate applications software from various layers of system software. This is a major improvement and exten- sion over the simpler two-level user/supervisor mechanism found in many systems. Software modules in a supervisor level are protected from modules in the application level and from software in less privileged supervisor levels.

7-1