88 Memory Systems Doug burger University of Wisconsin-Madison James R. Goodman 88.2 Memory Hierarchies 88.3 Cache memories 88.4 Parallel and Interleaved Memories Gurindar s. sohi 88.5 Virtual Memory University of Wisconsin-Madison 68.6 Research issues 88.1 Introduction A memory system serves as a repository of information(data)in a computer system. The processor [also called the central processing unit( CPU)] accesses(reads or loads) data from the memory system, performs compu tations on them, and stores(writes)them back to memory. The memory system is a collection of storage locations. Each storage location, or memory word, has a numerical address. A collection of storage locations forms an address space. Figure 88. 1 shows the essentials of how a processor is connected to a memory system via address, data, and control lines When a processor attempts to load the contents of a memory location, the request is very urgent. In virtuall all computers, the work soon comes to a halt(in other words, the processor stalls) if the memory request does not return quickly. Modern computers are generally able to continue briefly by overlapping memory requests, but even the most sophisticated computers will frequently exhaust their ability to process data and stall momentarily in the face of long memory delays. Thus, a key performance parameter in the design of any computer, fast or slow, is the effective speed of its leally, the memory system must be both infinitely large so that it can contain an arbitrarily large amount of information and infinitely fast so that it does not limit the processing unit. Practically, however, this is not possible. There are three properties of memory that are inherently in conflict: speed, capacity, and cost. In general, technology tradeoffs can be employed to optimize any two of the three factors at the expense of the third. Thus it is possible to have memories that are(1)large and cheap, but not fast; (2)cheap and fast, but small; or(3)large and fast, but expensive. The last of the three is further limited by physical constraints. A large-capacity memory that is very fast is also physically large, and speed-of-light delays place a limit on the speed of such a memory syste The latency(L)of the memory is the delay from when the processor first requests a word from memory until that word arrives and is available for use by the processor. The latency of a memory system is one attribut f performance. The other is bandwidth(BW), which is the rate at which information can be transferred from the memory system. The bandwidth and the latency are related. If R is the number of requests that the memory can service simultaneously, then: BW、R c 2000 by CRC Press LLC© 2000 by CRC Press LLC 88 Memory Systems 88.1 Introduction 88.2 Memory Hierarchies 88.3 Cache Memories 88.4 Parallel and Interleaved Memories 88.5 Virtual Memory 88.6 Research Issues 88.1 Introduction A memory system serves as a repository of information (data) in a computer system. The processor [also called the central processing unit (CPU)] accesses (reads or loads) data from the memory system, performs compu￾tations on them, and stores (writes) them back to memory. The memory system is a collection of storage locations. Each storage location, or memory word, has a numerical address. A collection of storage locations forms an address space. Figure 88.1 shows the essentials of how a processor is connected to a memory system via address, data, and control lines. When a processor attempts to load the contents of a memory location, the request is very urgent. In virtually all computers, the work soon comes to a halt (in other words, the processor stalls) if the memory request does not return quickly. Modern computers are generally able to continue briefly by overlapping memory requests, but even the most sophisticated computers will frequently exhaust their ability to process data and stall momentarily in the face of long memory delays. Thus, a key performance parameter in the design of any computer, fast or slow, is the effective speed of its memory. Ideally, the memory system must be both infinitely large so that it can contain an arbitrarily large amount of information and infinitely fast so that it does not limit the processing unit. Practically, however, this is not possible. There are three properties of memory that are inherently in conflict: speed, capacity, and cost. In general, technology tradeoffs can be employed to optimize any two of the three factors at the expense of the third. Thus it is possible to have memories that are (1) large and cheap, but not fast; (2) cheap and fast, but small; or (3) large and fast, but expensive. The last of the three is further limited by physical constraints. A large-capacity memory that is very fast is also physically large, and speed-of-light delays place a limit on the speed of such a memory system. The latency (L) of the memory is the delay from when the processor first requests a word from memory until that word arrives and is available for use by the processor. The latency of a memory system is one attribute of performance. The other is bandwidth (BW), which is the rate at which information can be transferred from the memory system. The bandwidth and the latency are related. If R is the number of requests that the memory can service simultaneously, then: BW (88.1) R L = Doug Burger University of Wisconsin-Madison James R. Goodman University of Wisconsin-Madison Gurindar S. Sohi University of Wisconsin-Madison