CONTROL MEMORYControl memory is a random access memory(RAM) consisting of addressable storage registers. It isprimarily used in mini and mainframe computers as atemporary storage for data. Access to control memorydata requires less time than to main memory; this speedsup CPU operation by reducing the number of memoryreferences for data storage and retrieval. Access isperformed as part of a control section sequence whilethe master clock oscillator is running.The control memory addresses are divided into twogroups: a task mode and an executive (interrupt) mode.Addressing words stored in control memory is via theaddress select logic for each of the register groups.There can be up to five register groups in controlmemory. These groups select a register for fetchingdata for programmed CPU operation or for maintenanceconsole or equivalent display or storage of data via amaintenance console or equivalent. Duringprogrammed CPU operations, these registers areaccessed directly by the CPU logic. Data routingcircuits are used by control memory to interconnect theregisters used in control memory.Some of the registers contained in a controlmemory that operate in the task and executive modesinclude the following:AccumulatorsIndexesMonitor clock status indicating registersInterrupt data registersCACHE MEMORYCache memory is a small, high-speed RAM bufferlocated between the CPU and main memory. Cachememory buffers or holds a copy of the instructions(instruction cache) or data (operand or data cache)currently being used by the CPU. The instructions anddata are copies of those in main memory.Cache memory provides two benefits. One, theaverage access time for CPU’s memory requests isreduced, increasing the CPU’s speed by providing rapidaccess to currently used instructions and data. Two, theCPU’s use of the available memory bandwidth isreduced. This allows other devices on the system busto use the memory without interfering with the CPU.Therefore, cache memory is used to speed up the flowof instructions and data into the CPU from mainmemory.This cache function is important because the mainmemory cycle time is typically slower than the CPUclocking rates. To accomplish this rapid data transfer,cache memories are usually built from the fasterbipolar RAM devices rather than the slowermetal-oxide-semiconductor (MOS) RAM devices.The RAMs used for cache memory may be eitherdynamic RAMs (DRAMs) or static RAMs (SRAMs).Cache memories are not part of the memory section andthey are transparent to programmers (i.e., not accessibleby machine instruction). Their size varies with the typeof computer; usually they are no more than 64K.PROPERTIES OF CACHE MEMORY.— Allcaches share the following properties:. A buffered memory or cache memory consists ofa small high-speed memory with main memoryinformation. This information may be addresses, data,or instructions. The speed of the small memory isusually on the order of one magnitude faster than mainmemory, and its capacity is typically one or two ordersof magnitude less than main memory.l A cache memory system requires an identifieror tag store to indicate which entries of main memoryhave been copied into it. Such an area is usuallyreferred to as the directory or tag store.l A cache memory requires a logical network andmethod of replacing old entries.. A cache memory uses timing and control.CACHE PROCESS.— The cache process takesplace when a CPU with a cache initiates a memoryreference. The address of the needed item is generatedand the cache is searched. The method of searchdepends on the type of cache mapping used by thecomputer system. We can generalize the cache processinto three areas as follows:l Searches—Reads from the cache directory witha hit indicating that the data from the requested addressis present, while a miss indicates that the data is notpresent.l Updates—Writes to the cache data as well as tothe directories with new informationol Invalidates—Writes only to the directories; thiseffectively removes an address that previously residedin cache.5-15
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business