Structural Aspects of the System/360 Model 85 II: The Cache

cache, a high-speed buffer establishing a storage hierarchy in the Model 85, is discussed in depth in this part, since it represents the basic organizational departure from other SYSTEM/~BO computers. Discussed are organization and operation of the cache, including the mechanisms used to locate and retrieve data needed by the processor. The internal performance studies that led to use of the cache are described , and simulated performance of the chosen configuration is compared with that of a theoretical system having an entire 80-nano-second main storage. Finally, the effects of varying cache parameters are discussed and tabulated. Among the objectives of the Model 85 is that of providing a SYSTEM/^^ compatible processor with both high performance and high throughput. One of the important ingredients of high through-put is a large main storage capacity (see the accompanying article in Part I). However, it is not feasible to provide a large main storage with an access time commensurate with the 80-nanosecond processor cycle of the Model 85. A longer access time can be partially compensated for by an increase in overlap, greater buffering, deeper storage interleaving, more sophistication in the handling of branches, and other improvements in the processor. All of these factors only partially compensate for the slower storage, and, therefore , we decided to use a storage hierarchy instead. The storage hierarchy consists of a 1.04-microsecond main storage and a small, fast store called a cache,l which is integrated into the CPU. The cache is not addressable by a program, but rather is used to hold the contents of those portions of main storage that are currently being used. Most processor fetches can then be handled by referring to the cache, so that most of the time the processor has a short access time. When the program starts operating on data in a different portion of main storage, the data in that portion must be loaded into the cache and the data from some other portion removed. This activity must take place without program assistance, since the Model 85 must be compatible with the rest of This paper discusses organization of the cache and the studies that led to its use in the Model 85 and to selecting of values for its parameters. the SYSTEM/360 line.