Memory System | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Microcomputer Memory

• Memory is an essential component of the microcomputer system.
• It stores binary instructions and datum for the microcomputer.
• The memory is the place where the computer holds current programs and data that are in use.
• None technology is optimal in satisfying the memory requirements for a computer system.
• Computer memory exhibits perhaps the widest range of type, technology, organization, performance and cost of any feature of a computer system.
• The memory unit that communicates directly with the CPU is called main memory.
• Devices that provide backup storage are called auxiliary memory or secondary memory.

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

Location

• 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.

Capacity

• 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.
  — e.g. 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.
  — e.g. 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. — e.g. 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.
  — e.g. cache

Performance

• 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?

Organization

• Physical arrangement of bits into words
• Not always obvious
  - e.g. interleaved