Digital Electronics: Principles, Devices and Applications

Computer Fundamentals 611CPURegistersCacheMemoryRandom Access Memory(RAM)Read Only Memory(ROM)Hard Drives/Removable DiscsVolatileMemoriesNon-VolatileMemoriesFigure 15.3Various types of memory present in a typical computer system.and CDs. Primary memory is described in this section, and secondary storage media are discussed inSection 15.10.15.4.1 Primary MemoryThe primary memory holds the program instructions for the program to be executed, the input data tobe processed and the intermediate results of any calculations when processing is being done. Primarymemory is also used for storing BIOS and start-up programs.When a program and data are entered into a computer, the control unit directs them to the primarymemory. Each program instruction and each data item is stored in a memory location that has a uniqueaddress. These data and instructions are held till new data items and instructions are written over them.Thus, the same data can be accessed repeatedly if so desired and the same instructions can be executedrepeatedly if so required. This is what is known as the stored program concept. The primary memoryof a computer further comprises process registers, random access memory (RAM), cache memory andread only memory (ROM). Process registers are memory cells built into the CPU that contain thespecific data needed by the CPU. Cache memory is basically a type of RAM memory.15.4.1.1 Random Access MemoryRAM is a read/write memory where the data can be read from or written into any of the memorylocations regardless of the order in which they are arranged. Therefore, all the memory locations ina RAM can be accessed at the same speed. RAM is used to store data, program instructions and theresults of any intermediate calculations during the execution of a program. Also, the same data can beread any number of times and different data can be written into the same memory location, with everyfresh data item overwriting the existing one. It is typically used for short-term data storage as it cannotretain data when the power is turned off.RAM is available in the form of ICs as well as in the form of plug-in modules. The plug-inmodules are small circuit boards containing memory ICs and having input and output lines connectedto an edge connector. They are available as single in-line memory modules (SIMMs) and dual in-linememory modules (DIMMs). More than one memory IC (or chip) can be used to build the RAM for