1 - Al Kossow's Bitsavers

AN INTRODUCTION TO CACHE MEMORY SYSTEMSAND THE TMS21 50As the typical operating speeds of processors have increased to provide for the ever increasing need for computingpower, the necessity of developing a memory hierarchy (the incorporation of two or more memorytechnologies in the same system) has become apparent. One of these memory technologies is selected on thebasis of fast access time (with associated high cost per bit) to allow minimum system cycle time. The othertechnologies are chosen with the lowest possible cost per bit relative to speed in order to achieve the maximumsystem memory capacity. In a system with a multiple level hierarchy, the speed/cost relationship depends uponthe frequency of access and the total memory requirement at that leveL By proper use of this hierarchy throughcoordination of hardware, system software, and in some cases user software, the overall memory system willreflect the characteristics that approximate the fast access time of the fast memory technology and the low costper bit of the low cost memory technology. Large computer systems have made use of this memory optimizationtechnique to maintain very large data bases and high throughput (see Figure 1). Many smaller processor systemsuse this technique to allow mass storage of data, where a tape or disk is the low cost memory and RAM (RandomAccess Memory) is the fast memory technology.Memory hierarchy is now extending to the RAM memory used in microcomputer systems because of the increasein processor speeds. Typically, Dynamic RAM (DRAM) is used as the bulk or main memory and HighSpeed Static RAM (HSS) serves as the fast access memory. This HSS RAM is usually 1 K to 8K words deep andserves as a fast buffer memory between the processor and the main memory. This small, fast buffer memory iscalled "cache" memory as it is the storage location for a carefully selected portion of the data from the mainmemory. The addresses for that portion of memory currently in the buffer memory is saved in the cache tag RAM(a small memory that is used to store the addresses of the data that has been mapped to cache) .co"';::;COE~....o.5enCo"';::;CO"~Q.c.•