Lecture 6 - 2/23/2012 1 Memory Memory CNIT 17600 Lecture 6...

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Unformatted text preview: 2/23/2012 1 Memory Memory CNIT 17600 Lecture 6 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 b Digital Circuit Design b Implementation via Gates and Latches b High-Level Interaction & Addressing 2 2/23/2012 2 ROM Read-Only-Memory (ROM) ROM does not need to be refreshed 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 3 Types of RAM 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 SRAM consists of circuits like the D flip-flop SRAM is very fast It is used to build cache memory 4 2/23/2012 3 Memory Packaging 5 EPROM 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 6 2/23/2012 4 Memory Hierarchy This storage organization can be thought of as a pyramid: 7 Memory Hierarchy The memory hierarchy involves registers, cache, main memory, and virtual memory Registers are storage locations available on the processor itself Very fast, but very costly Cache memory provides speed 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 8 2/23/2012 5 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 virtual memory Once the data is located, it and a number of its nearby data elements are fetched into cache memory 9 Memory Definitions A hit is when data is found at a given level A miss is when it is not found Hit rate is the percentage of time data is found at a given memory level The miss rate is opposite Miss rate = 1 - hit rate Hit time is the time required to access data at a given memory level Miss penalty is the time required to process a miss including the time required to replace a block of memory plus the time to deliver the data to the processor 10 2/23/2012 6 Memory Locality An entire block of data is copied after a hit because the principle of locality tells us that once a byte is accessed, it is likely that a nearby data element will also be needed soon There are three forms of locality: Temporal locality- Recently-accessed data elements tend to be accessed again Spatial locality - Accesses tend to cluster...
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This note was uploaded on 03/04/2012 for the course CNIT 176 taught by Professor Hansen during the Spring '09 term at Purdue University-West Lafayette.

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Lecture 6 - 2/23/2012 1 Memory Memory CNIT 17600 Lecture 6...

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