Chapter 6 - 1 Chapter 6 Chapter 6 Memory Chapter 6...

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Unformatted text preview: 1 Chapter 6 Chapter 6 Memory Chapter 6 Objectives ¡ Master the concepts of hierarchical memory organization organization. ¡ Understand how each level of memory contributes to system performance, and how the performance is measured. ¡ Master the concepts behind cache memory, virtual 2 memory, memory segmentation, paging and address translation. 2 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 3 organization. A clear understanding of these ideas is essential for the analysis of system performance. 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 4 every few milliseconds to prevent data loss. ¡ DRAM is “cheap” memory owing to its simple design. 3 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. 5 ¡ ROM is used to store permanent, or semi- permanent data that persists even while the system is turned off. 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. 6 ¡ Larger, (almost) permanent storage in the form of disk and tape drives is still further from the CPU. 4 6.3 The Memory Hierarchy ¡ This storage organization can be thought of as a pyramid: 7 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 8 number of its nearby data elements are fetched into cache memory. 5 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....
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This note was uploaded on 10/05/2010 for the course CGS CGS 3269 taught by Professor K during the Spring '10 term at University of Central Florida.

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Chapter 6 - 1 Chapter 6 Chapter 6 Memory Chapter 6...

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