Lecture 6

Lecture 6 - 10/13/2011 CNIT 17600 IT Architectures Memory...

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10/13/2011 1 CNIT 17600 IT Architectures Memory 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 2
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10/13/2011 2 6.1 Introduction Memory lies at the heart of the stored-program computer Previously, we detailed the components that we can use to build memory and the ways in which memory is accessed by various ISAs Logical Operations Digital Circuit Design Implementation via Gates and Latches High-Level Interaction & Addressing… 3 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) Static RAM (SRAM) DRAM consists of capacitors that slowly leak their charge over time. This requires a recharge every few milliseconds to prevent data loss DRAM is “cheap” because of its simple design 4
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10/13/2011 3 6.2 Types of Memory SRAM consists of circuits like the D flip-flop SRAM is very fast memory It is used to build cache memory ROM also does not need to be refreshed 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 5 6 Memory Packaging EPROM
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10/13/2011 4 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 close to the CPU, while larger and slower main memory is accessed through the data bus Larger, (semi-) permanent storage in the form of disk and tape drives is still further from the CPU 7 6.3 The Memory Hierarchy This storage organization can be thought of as a pyramid: 8
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10/13/2011 5 6.3 The Memory Hierarchy We are most interested in the memory hierarchy that involves registers, cache, main memory, and virtual memory Registers are storage locations available on the processor itself Very fast, but very costly. Virtual memory is typically implemented using a hard drive; it extends the address space from RAM to the hard drive Virtual memory provides more space; Cache memory provides speed 9 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 is sent to disk Once the data is located, then the data, and a number of its nearby data elements are fetched into cache memory 10
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10/13/2011 6 6.3 The Memory Hierarchy This leads us to some definitions. A
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Lecture 6 - 10/13/2011 CNIT 17600 IT Architectures Memory...

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