PowerPointPres_Ch6_ECOA2e

The Essentials of Computer Organization And Architecture

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Chapter 6 Memory
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2 Chapter 6 Objectives Master the concepts of hierarchical memory organization. Understand how each level of memory contributes to system performance, and how the performance is measured. Master the concepts behind cache memory, virtual memory, memory segmentation, paging and address translation.
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3 6.1 Introduction Memory lies at the heart of the stored-program computer. In previous chapters, we studied the components from which memory is built and the ways in which memory is accessed by various ISAs. In this chapter, we focus on memory organization. A clear understanding of these ideas is essential for the analysis of system performance.
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4 6.2 Types of Memory There are two kinds of main memory: random access memory, RAM, and read-only-memory, ROM . There are two types of RAM, dynamic RAM (DRAM) and static RAM (SRAM). Dynamic RAM consists of capacitors that slowly leak their charge over time. Thus they must be refreshed every few milliseconds to prevent data loss. DRAM is “cheap” memory owing to its simple design.
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5 6.2 Types of Memory SRAM consists of circuits similar to the D flip-flop that we studied in Chapter 3. SRAM is very fast memory and it doesn’t need to be refreshed like DRAM does. It is used to build cache memory, which we will discuss in detail later. ROM also does not need to be refreshed, either. In fact, it needs very little charge to retain its memory. ROM is used to store permanent, or semi- permanent data that persists even while the system is turned off.
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6 6.3 The Memory Hierarchy Generally speaking, faster memory is more expensive than slower memory. To provide the best performance at the lowest cost, memory is organized in a hierarchical fashion. Small, fast storage elements are kept in the CPU, larger, slower main memory is accessed through the data bus. Larger, (almost) permanent storage in the form of disk and tape drives is still further from the CPU.
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7 6.3 The Memory Hierarchy This storage organization can be thought of as a pyramid:
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8 6.3 The Memory Hierarchy To access a particular piece of data, the CPU first sends a request to its nearest memory, usually cache. If the data is not in cache, then main memory is queried. If the data is not in main memory, then the request goes to disk. Once the data is located, then the data, and a number of its nearby data elements are fetched into cache memory.
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9 6.3 The Memory Hierarchy This leads us to some definitions. A hit is when data is found at a given memory level. A miss is when it is not found. The hit rate is the percentage of time data is found at a given memory level. The
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This document was uploaded on 02/06/2011.

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PowerPointPres_Ch6_ECOA2e - Chapter 6 Memory Chapter 6...

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