Memory Unit/System

[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

• A 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

• A 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.