M.tech notes of Computer Science

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Wednesday, 26 October 2016

Fundamental of Memory

The Memory unit is an essential component in any digital computer. Memory is needed for storing programs and data. The total memory capacity of a computer can be visualized as being a hierarchy of components. The memory hierarchy system consists of all storage devices employed in computer system from the slow but high capacity auxiliary memory to a relatively faster main memory.
  •   The memory unit that directly communicate with CPU is called Main Memory. Only programs and data currently needed by the processor reside in main memory.
  • Devices that provides backup storage called auxiliary memory. The most common auxiliary devices used in computer system are magnetic disks and tapes. They are used for storing system programs, large data files, and other backup information.

  •        All other information is stored in auxiliary memory and transferred to main memory when needed.
     Very high speed memory is called Cache Memory. It is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate. CPU logic is usually faster than main memory access time. The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations. Cache organization is concerned with the transfer of information between main memory and CPU.

      Cache memory is: -  
·         Very small.
·         Relatively expensive.
·         and has very high access speed.
·         Cache memory holds those parts that are not presently used by the CPU.

     Main Memory: - Main memory is central storage unit in computer system. The principal technology used for the main memory is based on semiconductor integrated circuits. Integrated circuits RAM chips are available in two possible operating modes, static and dynamic. Main memory is: -
·         Fast.
·         Relatively large.
·         And used to store programs and data during the computer operations.

     The static RAM consist of internal flip-flops that store the binary information. The stored information remains valid as long as power is applied to the unit. Static RAM is easier to use and has shorted read and writes cycles.

     The dynamic RAM stores the binary information in the form of electric charges that are applied to capacitors. The capacitors are provided inside the chip by MOS transistors. The dynamic RAM offers reduced power consumption and larger storage capacity in single memory chip. The dynamic RAM’s are used to for implementing the main memory. Most of the computer systems are dynamic RAMs with improved performance characteristics such as multi-bank DRAM, synchronous DRAM, and Direct DRAM bus DRAM.

     Memory Connections to CPU: - RAM and ROM chips are connected to CPU through the data and address buses. The data bus of ROM has only one output capacity, whereas the data bus connected to RAMs can transfer information in both directions.

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