Besides processing data and getting the results for immediate use computer is used for storing data as well. Therefore, there are two types of memory in a computer system, one for storing data that are currently being handled by the CPU and another for storing data and results for future reference.

The storage system used for handling the running process is called primary memory. They are temporary in nature. And the storage which is used to store data and information for longer-term is called secondary memory.

The data and information held by the primary memory can be directly accessed by the CPU using data and address buses. However, the data stored in the secondary memory is to be fetched to the primary memory through I/O channel first and the CPU reads the data from the secondary memory via the primary.

There is a third type of memory as well which is known as internal process memory. These are placed either near to the CPU or inside the CPU itself.

Memory Representation

1 Byte = 8 Bits

1 KB = 1024 Bytes

1 MB = 1024 KB = 1048576 Bytes

1 GB = 1024 MB = 1073741824 Bytes

1 TB = 1024 GB = 1099511627776 Bytes

There are two types of memory:

There are mainly two types of memory:

  1. Primary Memory
  2. Secondary Memory

1. Primary memory

Primary memory is the main memory of computer which is used to store the information (programs and data) that the computer is currently using. Primary memory is categorized into two main types:

  1. Random Access Memory (RAM), and
  2. Read Only Memory (ROM)

A. Random Access Memory (RAM)

  • RAM (Random Access Memory) is the internal memory of the CPU for storing data, program and program result. It is read/write memory which stores data until the machine is working. As soon as the machine is switched of, data is erased.
  • Access time in RAM is independent of the address that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive.
  • RAM is volatile, i.e. data stored in it is lost when we switch of the computer or if there is a power failure. Hence a backup uninterruptible power system (UPS) is often used with computers.
  •  RAM is small, both in terms of its physical size and in the amount of data it can hold.

RAM is of two types

  • Static RAM (SRAM)
  • Dynamic RAM (DRAM)

i) Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power is being supplied. However, data is lost when the power gets down due to volatile nature. SRAM need not have to be refreshed on a regular basis in order to maintain the data. SRAM is used as cache memory and has very fast access.

SRAM uses multiple transistors (four to six), for each memory cell. It does not have a capacitor in each cell.  

Types of the Static RAM:

a. Asynchronous SRAM

It performs operations without the use of a system clock. It makes use of three signals for working, i.e. chip select (CS), write enable (WE) and output-enable (OE).

b. Burst SRAM

It works in association with the system clock. Therefore, it is also known as synchronous SRAM. It is used with high-speed applications as the read and writes cycles are synchronized with the clock cycle of the processor. The access waiting time gets reduced after the read and write cycle are synchronized.

c. Pipeline Burst SRAM

It uses the pipeline technology in which a large amount of data is broken up into different packets containing data. These packets are arranged in a sequential manner in a pipeline form and are sent to the processor simultaneously. It can handle a large amount of data at a very high speed. In fact, it is the fastest form of SRAM.

Characteristic of the Static RAM:

  • It has long life
  • There is no need to refresh
  • Faster
  • Used as cache memory
  • Large size
  • Expensive
  • High power consumption

ii) Dynamic RAM (DRAM):

DRAM, unlike SRAM, must be continually refreshed (recharged) in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small.

It uses transistors and capacitors. The transistors are arranged in a matrix of rows and columns. The capacitor holds the bit of information 0 and 1. The transistor and capacitor are paired to make a memory cell. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

Type of DRAM

i) Synchronous DRAM

It performs operations in association with the system clock cycle. It is used with the processor for storing data continuous manner. The continuous form of data storage helps in processing a greater number of instructions per unit time which ultimately increases the speed of data transfer.

ii) Rombus DRAM

It contains multiple addresses and data line that helps in increasing the data access speed. It is faster than Synchronous DRAM.

iii) Extended Data Out DRAM

It can access more than a bit of data at a time which helps in increasing data access rate. It allows to perform various tasks at a time such as read and write.

iv) Fast Page Mode DRAM

It makes use of paging in which read and write operation is performed by selecting the address of data from rows and columns of the matrix. Use of paging doesn’t allow the use of bus at the memory of 66 MHz because of which reading and writing data from matrix consumes a lot of time.

Characteristics of the Dynamic RAM:

  • It has short data lifetime
  • Need to be refreshed continuously
  • Slower as compared to SRAM
  • Used as RAM
  • Lesser in size
  • Less expensive
  • Less power consumption

B. Read Only Memory (ROM)

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture. A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Following are the various types of ROM:

i) Programmable ROM (PROM)

It is a memory chip on which write operation can be performed only once. The data on this ROM is written permanently and cannot be erased afterwards. The writing process on such ROM is called ‘Burning ROM’. It is mostly used in video games and electronic dictionaries.

ii) Erasable Programmable ROM (EPROM)

In such type of ROM, data can be erased or destroyed using Ultra-Violet (UV) rays. In such, the facility of changing the content is available. It facilitates the storage of data for longer terms.

iii) Electrically Erasable Programmable ROM (EEPROM)

It is the type of ROM in which data can be erased or destroyed electrically with the means of electric charge. Here, data can be written or read at the rate of one bit per unit time which makes it slow.

iv) Flash ROM (FROM)

It is a variation of EEPROM that uses floating gate transistors which can store data for a longer period of time. It is mainly used in mobile phones, digital cameras, etc. It is the fastest form of ROM. It uses continuous memory cells for storing data. It is of two types.

a. NAND Flash ROM
b. NOR Flash ROM

2. Secondary Memory

Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and floppy disks. Secondary memory (or secondary storage) is the slowest and cheapest form of memory. It cannot be processed directly by the CPU. It must first be copied into primary storage (also known as RAM ). Secondary memory devices include magnetic disks like hard drives and floppy disksoptical disks  such as CDs and CDROMs ; and magnetic tapes, which were the first forms of secondary memory.

Types of Secondary Memory:

  • Magnetic Storage System
  • Optical Storage System
  • Magneto-Optical System
  • Solid State Drive (SSD)

A. Magnetic Storage System

1. Magnetic Tapes

They are plastic tapes with a magnetic coating that are used for data storage. They are similar to normal recording tapes which can be used for storing audio and video. Data can be accessed sequentially.


  • low cost and high storage
  • easily transportable
  • easy to handle and store


  • low data transmission due to a sequential transmission
  • low life duration
  • required protected environment for storage

The features of magnetic tape are:

  • Inexpensive storage device
  • Can store a large amount of data
  • Easy to carry or transport
  • Not suitable for random access data
  • Slow access device
  • Needs dust prevention, as dust can harm the tape
  • Suitable for back-up storage or archiving

2. Magnetic Disks

It is a flat disk covered with magnetic coating for holding information. It is used to store digital information in the form of small and magnetized needles. It is large in storage size and cheaper than RAM. It allows random access to data and provides the facility of erasing and recording data as many times as required.


  • High storage capacity and low cost
  • Easy and direct access to data
  • Less prone to corruption of data compared to magnetic tapes


  • Prone to physical damage since it has a physical moving part
  • Slower than SSDs

Features of magnetic disk:

  • Cheap storage device
  • Can store a large amount of data
  • Easy to carry or transport
  • Suitable for frequently read/write data
  • Fast access device
  • More reliable storage device
  • To be prevented from dust, as the read/write head flies over the disk. Any dust particle in between can corrupt the disk

B. Optical Storage System

This storage system uses the laser light as the optical medium to retrieve as well as record data.


They are non-volatile and reliable compare to magnetic tapes.


  • Large storage capacity
  • Longer life-span than magnetic tapes
  • Low ‘cost-per-bit’ for storage
  • Portable


  • Prone to scratches which could disturb or totally prevent the driver to perform R/W operations

C. Magneto-Optical System

It includes the features of both optical and magnetic disks. Its main objective is to store the data on personal computers for a longer period of time. It performs R/W operation by making use of laser and optical technology. It has ferromagnetic particles enclosed in a plastic coating. The laser beam used for R/W operation gets reflected due to magnetic surface of the disk.

It is slower in speed but its manufacturing cost is high.

D. Solid State Drive (SSD)

It is the device that contains all the properties of a hard drive to store the data and uses the solid-state memory which has no moving part. It uses semiconductors to store data. Its main principle is to process data electronically. Since it has no mechanical moving part, the data access time is faster than that in HDD.


  • Better performance
  • high access speed, random access of data, faster R/W operation
  • Low power consumption
  • Highly reliable
  • Small dimension/ compact size


  • High cost
  • Lower capacity
  • Low storage density
  • Vulnerable – data loss, affected by power loss, electrostatic discharges and magnetic fields