Microprocessor Theory and Applications with 68000/68020 and Pentium

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MICROPROCESSOR MEMORY ORGANIZATION

In this chapter we describe concepts associated with memory organization in typical microprocessors. Topics include main memory array design, memory management, and cache memory concepts.

3.1 Introduction

A memory unit is an integral part of any microcomputer, and its primary purpose is to hold instructions and data. The major design goal of a memory unit is to allow it to operate at a speed close to that of a microprocessor. However, the cost of a memory unit is so prohibitive that it is practically not feasible to design a large memory unit with one technology that guarantees high speed. Therefore, to seek a trade-off between the cost and the operating speed, a memory system is usually designed with different technologies, such as solid state, magnetic, and optical. In a broad sense, a microcomputer memory system can be divided into three groups:

1. Microprocessor memory

2. Primary or main memory

3. Secondary memory

Microprocessor memory comprises to a set of microprocessor registers. These registers are used to hold temporary results when a computation is in progress. Also, there is no speed disparity between these registers and the microprocessor because they are fabricated using the same technology. However, the cost involved in this approach limits a microcomputer architect to include only a few registers in the microprocessor.

Main memory is the storage area in which all programs are executed. The microprocessor can directly access only ...