中山大学软件学院专业课计算机组成原理课件第3章.pptVIP

Characteristics of Memory Systems Location CPU Internal External Access Methods (1) Sequential Start at the beginning and read through in order Access time depends on location of data and previous location e.g. tape Direct Individual blocks have unique address Access is by jumping to vicinity plus sequential search Access time depends on location and previous location e.g. disk Access Methods (2) Random Individual addresses identify locations exactly Access time is independent of location or previous access e.g. RAM Associative Data is located by a comparison with contents of a portion of the store Access time is independent of location or previous access e.g. cache Performance Access time Time between presenting the address and getting the valid data Memory Cycle time Time may be required for the memory to “recover” before next access Cycle time is access + recovery Transfer Rate Rate at which data can be moved Physical Types Semiconductor RAM Magnetic Disk Tape Optical CD DVD Others Hologram The Bottom Line How much? (Capacity) Greater capacity, smaller cost per bit Greater capacity, slower access time How fast? (Access time) Faster access time, greater cost per bit How expensive? (Cost) Memory Hierarchy Registers In CPU Internal or Main memory May include one or more levels of cache “RAM” External memory Backing store Hierarchy List Registers L1 Cache L2 Cache Main memory Disk Optical Tape Memory Hierarchy - Diagram Hierarchy List Cache Small amount of fast memory Sits between normal main memory and CPU May be located on CPU chip or module Content addressable memory What makes cache “special”? Cache is not accessed by address; it is accessed by content. For this reason, cache is sometimes called content addressable memory or CAM. Terminology Hit: The requested data resides in a given level of memory. Miss: The requested data is not found in the given level of memory. Hit Rate: The percentage of memory accesses found in a given level o