The memory system can be characterised with their Location, Capacity, Unit of transfer, Access method, Performance, Physical type, Physical characteristics, Organisation.


  • Processor memory: The memory like registers is included within the processor and termed as processor memory.
  • Internal memory: It is often termed as main memory and resides within the CPU.
  • External memory: It consists of peripheral storage devices such as disk and magnetic tape that are accessible to processor via i/o controllers.


  • Word size: Capacity is expressed in terms of words or bytes.
    • The natural unit of organisation
  • Number of words: Common word lengths are 8, 16, 32 bits etc.
    • or Bytes

Unit of Transfer

  • Internal: For internal memory, the unit of transfer is equal to the number of data lines into and out of the memory module.
  • External: For external memory, they are transferred in block which is larger than a word.
  • Addressable unit
    • Smallest location which can be uniquely addressed
    • Word internally
    • Cluster on Magnetic disks

Access Method

  • Sequential access: In this access, it must start with beginning and read through a specific linear sequence. This means access time of data unit depends on position of records (unit of data) and previous location.
    • eg. tape
  • Direct Access: Individual blocks of records have unique address based on location. Access is accomplished by jumping (direct access) to general vicinity plus a sequential search to reach the final location.
    • eg. disk
  • Random access: The time to access a given location is independent of the sequence of prior accesses and is constant. Thus any location can be selected out randomly and directly addressed and accessed.
    • eg. RAM
  • Associative access: This is random access type of memory that enables one to make a comparison of desired bit locations within a word for a specified match, and to do this for all words simultaneously.
    • eg. cache


  • Access time: For random access memory, access time is the time it takes to perform a read or write operation i.e. time taken to address a memory plus to read / write from addressed memory location. Whereas for non-random access, it is the time needed to position read / write mechanism at desired location.
    • Time between presenting the address and getting the valid data
  • Memory Cycle time: It is the total time that is required to store next memory access operation from the previous memory access operation.

Memory cycle time = access time plus transient time (any additional time required before a second access can commence).

  • Time may be required for the memory to “recover” before next access
  • Cycle time is access + recovery
  • Transfer Rate: This is the rate at which data can be transferred in and out of a memory unit.
    • Rate at which data can be moved
    • For random access, R = 1 / cycle time
    • For non-random access, Tn = Ta + N / R; where Tn – average time to read or write N bits, Ta – average access time, N – number of bits, R – Transfer rate in bits per second (bps).

Physical Types

  • Semiconductor
    • RAM
  • Magnetic
    • Disk & Tape
  • Optical
    • CD & DVD
  • Others
  • Bubble
  • Hologram

Physical Characteristics

  • Decay: Information decays mean data loss.
  • Volatility: Information decays when electrical power is switched off.
  • Erasable: Erasable means permission to erase.
  • Power consumption: how much power consumes?


  • Physical arrangement of bits into words
  • Not always obvious

e.g. interleaved