Introduction of Memory Units

  • Memory Units are also known as Memory Systems.
  • Computer memory is the physical device/component where the computer holds data, information, and programs either temporarily or permanently that are in use. 
  • When the data or information is kept in memory the CPU can access it much more quickly. The CPU is constantly using the memory system during processing time.
  • Memory Write :
    • The act of entering data into a storage location is called a Memory Write operation.
    •  The transfer of new information to be stored in memory is called a write operation.
    • The memory write operation transfers the content of a data register to a memory word M selected by the address. Suppose that the data of register R1 is to be written to the memory at the address provided in MAR. Thus, the write operation can be stated as :
      Write : [MAR] ← R1
  • Memory Read :
    • The act of retrieving data from a storage location is called a Memory Read operation.
    • The transfer of information from memory to the outside world through the I/O Interface is called a read operation. 
    • During the memory transfer, the memory unit receives the address from a register, called the memory address register designated by MAR. The data is transferred to another register, called the data register designated by DR. The read operation can be stated as:
      Read: DR ←  [MAR]
  • Memory Transfer :
    • Data and instructions are moved, to and from memory, in bunches of word length.
    • These kinds of transfers are achieved via a system bus. It is necessary to supply the address of the memory location for memory transfer operations.

Definition

  • Memory Unit is the storage component of a computer system where all the data, information, programs, etc. are stored in the form of binary digits (combination of 0’s & 1’s) either temporarily or permanently.

Features

  • The Memory unit is a very important & essential component of a computer system.
  • A typical computers contain both semiconductor as well as magnetic memory.

Types

[A] On the basis of access time, storage capacity and cost-per-bit of storage, the memory devices are categorised into two main groups –

(1.) Primary Memory : –
  • It is a read/write memory used to store both the program and data.
  • It stores data temporarily i.e. volatile  memory.
  • They are comparatively faster in access time,  smaller in storage capacity, and costly or high cost/bit in making it with respect to secondary memory.
  • Examples are – Main memory or RAM.
(2.) Secondary or Auxiliary Memory : –
  • It stores data permanently i.e. non-volatile memory.
  • It is considered as external memory.
  • Magnetic and Optical memory belongs to this category.
  • Secondary memory is mainly used to store bulk data, programs and other information hence called mass storage device.
  • It has much larger storage capacity than main memory but is slower.
  • It stores system software such as OS, compiler, assembler and useful application packages, large data files etc.
  • Secondary memory cannot be accessed directly by the CPU. Its information is first transferred to the main memory and then the information can be accessed by the CPU.
  • Secondary storage systems offer large storage capacities, low cost per bit and medium access times.
  • The access time of mass storage devices such as hard disks are measured in milliseconds (ms).
  • Examples are – Hard disk, floppy disk, Flash or Pen Drive, CD ROMs, DVD ROMs, Magnetic tapes etc.
(3.) CPU/Processor/Register Memory : –
  • It is the smallest and fastest memory in a computer.
  • It is not the part of the main memory.
  • Its access time is 0.5-2 ns (nanosecond).
  • It is located in the CPU and associated with the processor of the computer.
  • They are the smallest, temporarily data holding elements that  holds frequently used data, instructions, and memory address that are to be used by the processor immediately.
  • These are the small set of high speed memories which are internal to a processor.

[B] On the basis of permanence of data storage, the memory devices are categorised into two main groups –

(i)  Volatile Memory
  • A memory whose contents are disappeared/removed/deleted automatically when power supply is OFF.
  • It requires power to maintain the stored information.
  • Examples are – RAM/Main memory, Cache memory, CPU memory etc.
(ii) Non-volatile Memory
  • A memory whose contents are not disappeared/removed/deleted automatically when power supply is either ON/OFF.
  • It can retain the stored information even when not powered.
  • Here, the contents can be removed when user wants.
  • Secondary storage belongs to this category.
  • Examples are – Hard disk, ROM, Flash memory, CD/DVD/Blu-ray Disc, Magnetic tape, Punched cards etc.

[C] On the basis of detachment of data storage devices easily from the computer, the memory devices are categorised into two main groups –

(a)  Removable Memory/Storage Device
  • storage device that can be removed from a computer while the system is running/ON.
  • Removable media makes it easy for a user to move data from one computer to another easily.
  • Removable devices are mainly used for back-up purposes having higher storage density and higher transfer rate.
  • Examples are – Flash memory or Pen drive, CD/DVD/Blu-ray disk, Memory Cards, Floppy disks, External Hard disk, Digital Camera, Smartphones etc. 
(b) Non-Removable Memory/Storage Device
  • storage device that can’t be removed from a computer while the system is running/ON.
  • Examples are – Internal Hard disk, Main memory/RAM, Cache memory, Register memory etc. 

[D] On the basis of different material & technology used in the construction of memory devices,   they are categorised into two main groups –

(a) Magnetic Memory
  • A memory is coated with magnetic material paste such as iron oxide, is called magnetic memory.
  • It stores data in the form of tracks, spots and sectors.
  • The nature of data storage medium is magnetic in it. 
  • The data accessing mode is Sequential or Random.
  • They are normally non-volatile memory.
  • The making average cost Rs/bit is 10-6.
  • Its data access time is 10-1 sec.
  • Examples are – Magnetic disks(Hard disk, Floppy disk, Zip disk, Magnetic tape, Magnetic drums etc), 
(b)  Semiconductor  Memory
  • When a memory is constructed of a semi-conductor material(such as Silicon) is called a semi-conductor memory.
  • It is small in size, low in power, and less in access time.
  • The major components of semiconductor memory is of various kinds of electronic devices such as diodes, transistors, and integrated circuits.
  • Semiconductor memories are used mainly for primary storage i.e. it stores programs and data which are currently needed by the CPU.
  • The semiconductor memory is an electronic & static(not rotating like hard disk or optical disk) device. There are no moving parts in it like head in hard disk.
  • The semiconductor memory is faster, compact and lighter.
  • It consumes less power.
  • Primary memory belongs to this category.
  • They are normally volatile memory.
  • The nature of data storage medium is electronic in it.
  • The data accessing mode is Random or Direct.
  • The making average cost Rs/bit is 10-2.
  • Its data access time is 10-8 sec.
  • All computers except very small computers contain both semiconductor as well as magnetic memory.
  • All modern computers use semiconductor memory as in the form of its main memory(primary memory).
  • Semiconductor memory use Random access method to access the data for reading and writing.
  • Examples are – RAM & ROM.
(c)  Optical Memory
  • Optical memory is a memory in which data is written and read with the help of special light/optic called laser.
  • Optical storage devices offer a higher storage density at a lower cost.
  • A CD/DVD-ROM can be used as a major optical storage device.
  • Many software companies offer both operating system and application/driver software on CD/DVD-ROMs today.
  • This technology has been the main catalyst for the development of multimedia in computer fields.
  • The nature of data storage medium is optical in it.
  • The data accessing mode is Random.
  • They are normally non-volatile memory.
  • The making average cost Rs/bit is 10-7.
  • Its data access time is 1 sec.
  • Examples are – CDs and DVDs RAM or ROM, write-once read-many (WORM), Blu-ray disc etc.

[E] On the basis of priority use of memory while running the computer,   they are categorised into two main groups –

(a)  Internal/Fixed Memory
  • A memory that stores small amounts of necessary data that can be accessed quickly while the computer is running.
  • This memory provides data quickly to the current program.
  • Examples are – RAM & ROM.
(b) External Memory
  • A storage device that can retain or store data permanently and is outside the system/CPU.
  • External memory enables users to store data separately from a computer at a relatively low cost.
  • It increases storage capacity reliably.
  • Examples are – Flash memory or Pen drive, CD/DVD/Blu-ray disk, Memory Cards, Floppy disks, External Hard disk, online cloud storage etc.

Function

  • There are two main functions of the memory in computer :
    • To store programs, data, instructions and information into the computer.
    • All the intermediate results of computation and outputs from the program are stored in the memory.

Memory Hierarchy

  • The arrangement/organization of memories in certain specific order (the cost versus access time)in a computer system is known as memory hierarchy.
  • This includes CPU registers, Cache memory, Main memory and various secondary storage devices such as Hard disk, Magnetic tape etc.
  • The memory hierarchy is organized in such a way to minimize the cost, without compromising the overall speed of access.

Primary/Main Memory

Cache Memory


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