Access efficiency e, of a two level memory system as a function of hit ratio H for various values of R=tA2/tA1. – O.A. Ofem and B.I. Ele
Every computer system contains a variety of devices to store the instructions and data required for its operation. These storage devices plus the algorithms (either implemented by hardware or software) needed to control or manage the stored information constitute the memory system of the computer. In general, it is desirable that processors should have immediate and uninterrupted access to memory, so the time required to transfer information between a processor and memory should be such that the processor can operate at; or close to, its maximum speed, unfortunately, memories that operate at speeds comparable to processor speeds are relatively costly. It is not flexible (except for very small systems) to employ a single memory using just one type of technology. Instead the stored information is distributed in a complex fashion over a variety of different memory units with very different physical characteristics. The memory components of a computer system can be divided into three main groups:(1) Internal processor memory. This comprises a small set of high speed registers used as a working memory for temporary storage of instructions and data.(2) Main memory (also called primary memory) this is a relatively large fast memory used for program and data storage during computer operation. It is characterized by the fact that locations in main memory can be accessed directly and rapidly by the CPU instruction set. The principal technology used for main memory is based on semiconductor integrated circuits (ICS). (3) Secondary memory (also called auxiliary or backing memory). This is generally much larger in capacity but also much slower than main memory. It is used for storing system programs, large data files and the like which are not continually required by the CUP; it also serves as an overflow memory when the capacity of the main memory is exceeded. Information in the secondary storage is accessed indirectly Via input – output programs that first transfer the required information to main memory. Representative technologies used for secondary memory are magnetic disks and tapes.