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1.4.2 Types of Memory

            The memory unit consists of the following types of memory:

            A. Primary Memory
            Primary memory (also called main memory) is used to store data and instructions to be processed by the CPU. Data
            and instructions may be stored (written into memory) and retrieved (read from memory) for processing. Primary
            memory is mainly of two types:
              Random Access Memory (RAM): RAM is a random access memory in which data can be read from and written onto any
            random memory location. It is also commonly known as main memory or read/write memory. As the primary memory stores
            data and instructions associated with programs currently being executed by the CPU, the data is not stored permanently.
            The data and instructions are retained in the RAM as long as the computer remains connected to the power supply. RAM
            contents are lost when the power supply is switched off. So, RAM is also known as volatile memory.
              Read Only Memory (ROM): Data is written in Read Only Memory (ROM) using specialised hardware. ROM is a
               non-volatile memory that stores the data permanently, i.e. the contents are not lost even when the power supply is
               turned off. The data access from ROM is comparatively slower as compared to RAM. Data and instructions stored in
               ROM at the time of manufacturing are rarely to be changed. A ROM variant, Erasable Programmable ROM (EPROM),
               allows data in EPROM to be overwritten using a specialised hardware mechanism. When a computer is switched on,
               instructions in ROM gets executed to make the computer ready for use. The program in ROM that initiates the process
               of bringing up the computer for use is called the bootstrap program.



                   Various types of ROM are PROM, EPROM, and EEPROM.


            B. Cache Memory

            The processing speed of a CPU is much higher compared to the rate at which a CPU can exchange data with RAM. Thus,
            the execution speed of instructions involving read and write access from and to memory are limited by the speed of
            memory. Consequently, the overall processing speed of the computer system becomes slow. To overcome the gap
            between the CPU speed and the rate at which data is exchanged between the CPU and memory, high-speed cache
            memory is placed between the CPU and primary memory (Fig 1.5). The cache is used to store the frequently used
            instructions and data. Thus, the average time required to access data from primary memory is reduced. Whenever the
            CPU needs some data/ instruction, it first checks for its availability in the cache. Only if the required data/instruction is
            not found in the cache (cache miss), it is read from the main memory.


                                               Cache
                                               Memory


                                                                   Primary                 Storage/
                               CPU                                                         Secondary
                                                                   Memory
                                                                                           Memory





                                                       Fig 1.6: Cache Memory










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