Computer Memory Types – Classification of Storage/Memory Unit

Computer Memory Types – Classification of Storage/Memory Unit: The storage unit of all computers is comprised of the following two types of storage:

  • Primary Storage (Primary Memory)
  • Secondary Storage (Seconding Memory)

Primary Memory:

It is commonly known as main memory of computer. It is mainly used to gram, instructions and data, and intermediate results of processing of the jobs when the computer is currently working on.

The primary memory is directly accessible to the CPU because it is found inside the CPU and that is why it is known as Internal Memory.

Primary memory or main memory is basically of two types:

  • RAM (Read/Write Memory)
  • ROM (Read only Memory)


The speed of the CPU is extremely high as compared to access time of mam memory.

The disk processor speed mismatch to a large extent because the rate of data fetching by the CPU from the main memory is about 100 times faster than that from a high-speed secondary storage like disk.

Cache memory

However, even with the use of main memory, the memory processor speed mismatch with which the CPU can process instructions because there is a 1 to 10 speed mismatch between the processor and the memory.

That is the rate at which data can be fetched form memory is about 10 times slower than the rate at which CPU can process data.

Hence in many situations the performance of a processor gets limited due to the slow speed of main memory.

The performance of a processor can be greatly improved by minimizing the memory-processor speed mismatch. Cache memory is commonly used for minimizing the memory processor mismatch.

Cache memory is extremely fast, small memory between CPU and main memory whose access time is closer to the processing speed of the CPU.

It acts as a high-speed buffer between CPU and main memory and is used to temporarily store very active data and instructions during processing.

Cache memory is of two kinds—

  • Level l cache i.e., 1,1 cache and Level 2 cache i.e., L2 cache.

Level 1 Cache (L1):

It is the cache built into the CPU itself and stores the information to be accessed frequently so as to save the time compared to having get if from the RAM.

It is also known as Internal cache. Caching will store any code that has been read and keeps it available for the CPU to use.

This eliminates the need to wait for fetching the data from RAM.

Level 2 Cache (L2):

L1 is also known as External cache. It is additional cache can be added t most computers, depending on the motherboard.

This cache is mounted directly on the mother board outside the CPU.

L2 cache is quite a bit larger than L1 cache.


It is a read/write (R/W) memory which is volatile.  This means when power is turned off, all the contents are destroyed.

This is known as random access memory, a type of computer memory that can be accessed randomly, that is, any byte of memory can be accessed without touching the preceding bytes.

RAM Random Access Memory

Physical, this memory consists of some integrated chips (IC chips) either on the motherboard or on a small circuit board attached to the mother board.

A computer’s motherboard is designed in a manner that its memory capacity can be easily enhanced by adding more memory chips.

There are two different types of RAM:

  1. DRAM (Dynamic Random Access Memory)
  2. SRAM (Static Random. Access Memory)


Dynamic RAM consists of a transistor and capacitor that is capable of storing an electric charge.

Depending upon switching action of the transistor, the capacitor either contains no charge (0 bit) or does hold a charge ( I bit).

DRAMs are smaller in physical size, cheaper and slower memories.

Types of DRAM

  1. EDO DRAM: It stands for extended data output dynamic random access memory. EDO DRAM cells keeps its data valid until it receives an additional signal. It is a dual pipeline architecture, which allows the memory controller to simultaneously read new data while discharging the old.
  2. SDRAM (Synchronous DRAM): SDRAM synchronizes itself with the microprocessor clock speed allowing faster access to memory. These chips are designed so that memory chips could operate in synchronization with the host microprocessor and deliver data in each clock cycle.
  3. RDRAM (Rambus DRAM): The RDRAM is fairly fast, and it has tried to address some of the complex electrical and physical problems involved with memory.
  4. DDR SDRAM (Double Data Rate SDRAM): Unlike SDRAM memory that supports one operation in each clock cycle, DDR SDRAM memory can do two operations per clock cycle, there by doubling the memory bandwidth.


Static RAM is also volatile but as long as they are supplied with power, they need no special regenerator circuits to retain the stored data.

The static RAM consists eventually of internal flip-flops that store binary information.

The stored information remains valued as long as power is applied to the unit.

The static RAM is easier to use as compared to dynamic RAM.


As the name indicates, a read only memory (ROM) is a memory unit that performs the read operation only, it does not have a write capability.

read only memory

This implies that the binary information stored in a ROM is made permanent during the hardware production of the unit and cannot be altered by writing different words into.

It is a non-volatile memory which means if retains its contents even if the power is turned off.

ROM’s are used for applications in which it is known that the information never needs to be altered.

For example monitor program controlling a machine.

There are various types of ROM which are given below:

  • Programmable Read Only Memory (PROM): This is a type of ROM which can be programmed even after its manufacture. The programming can be done by special circuitry. Each bit in PROM can be individually programmed to a 1 or 0 by burning out a fusible link. A link once fused is permanent and cannot be changed. The main disadvantage of PROM is that it cannot be reprogrammed.
  • Erasable Programmable Read Only Memory (EPROM): This is a type of ROM is better than PROM in the sense that is can be reprogrammed. To erase the data, EPROM is exposed to ultraviolet light. Once erased, EPROM can be reprogrammed using electrical Usually the programming of EPROM is done with the help of electronic device called PROM programmes. The erasing time of an EPROM is 10 to 30 minutes.
  • Electrically Erasable Programmable Read Only Memory (EEPROM): This ROM is also called Electrically Altered ROM (EAROM). It can be repeatedly erased and programmed by electrical signals. Now a days it is being used as a backup to RAM memory. During a power failure the contents of RAM are lost. When power is returned, the contents of EEPROM are copied back into the RAM and the computer resumes normal working as if nothing had happened.


Secondary storage/ memory is used to store data permanently. It is also known as auxiliary memory.

It is basically developed to take care of limitations of primary storage which has limited capacity to store data because of its cost.

That is why secondary memory is developed for future use. The most commonly used secondary memory devices are magnetic tapes, hard disks, magnetic drums, floppy disk etc.

Secondary storage is cheaper then primary storage and can retain the information even the computer system is switched off.

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Hard Disk Drive

A hard disk consists of a stack of disks on a common spindle, each surface of which is coated with the magnetic material.

Both surfaces of each disk are used to record data except for the top and bottom disks where only the inner surfaces are used for each recording surface there is a read/ write head.

A set of read/write heads joined to a common arm in a comb like structure moves over the surface of the spinning disk to access concentric tracks of data. The arrangement is shown in figure below:

hard disk assembly


The disk pack is rotated at a high speed varying from 3600 rpm to 7200 rpm.

The speed or rotation causes the r/w head to float above the surface with a gap of 0.064 mm approximately.

When the constant speed is reached, all arms move together to position the head on a required track on surfaces.

However only one head is selected by circuitry at a given time so that data is actually accessed only though the selected head.

The most popular type of hard disk in use in Winchester disk. This type of hard disk used Winchester technology which was developed to reduce the problem of disk crash.

It is kept with in a hermetically sealed chamber. The surface of each disk is coated with a special lubricant which helps the head mechanism to take off and land on the disk without scratching the surface.

The time to access data from the disk involves seek time and rotational latency. The two terms are defined below:

  • Seek Time: This is the time require to position the read/ write head to the desired track. An average seek time on a modem hard disk is 8-10 to milliseconds.
  • Rotational Latency: This is rotational delay required for the desired sector to come underneath the read/write head. The average rotational latency on the modern hard disk is 2 to 6.7 ms.

The average rotational latency is taken to be half the time for one complete revolution of the disk.

Access Time = Seek Time + Latency Time.

Therefore the average access time of a hard disk is 12 to 19 ms.

The capacity of the hard disk drive is the number of bytes it can hold.

It is an electromechanical device which means it is constructed from electronic (IC’S, transistors, resistors etc.) and mechanical (motors, latches, mechanical switches etc. components).

The read/write head of a hard disk device never touches the surface of disk during rotation, it results on a head crash.

CD/DVD Drive

CD-ROM and DVD are two basic forms of optical storage used nowadays. These devices are called optical devices because they store data on a reflective surface so that it can be read by a beam of laser light.

CD DVD Memory

These disks are plastic disks within a layer of aluminum stamped on the surface. A layer of transparent plastic is further deposited on the disk. Compact disk storage was derived from the same media that we use from audio compact disks.

The storage space averages around 650 to 700 MB of data per disk. The data on the CD -ROM is read in the for of a continuous spiral just like a home record player.

The CD-ROM surface contains data recorded in the form of continuous spiral having pits and land. These pits and lands are small indentations and spaces between them. A pit scatter the light and land reflects the light.

A laser beam is projected by a focusing coil on the surface of the disk. if the beam strikes a land then it is reflected back otherwise it gets scattered. the reflected beam is pressed through a prism to strike a detector, a light sensing diode.

The light pulses are converted into 0′s and l’s. The CD-ROM contains data oil both sides of same disk. Most of the software we get in the market are distributed in the form of CD.

DVD(Digital Versatile Disk) was the development for a compact digital video format that is optical information storage and retrieval device.

It stores information through the Digital Recording method and it uses the technique of laser beam both for reading and writing data as well as graphical images on the round discs.

DVD-ROM is capable of storing a full length movie on the single disk.

Pen Drive

Pen drive is a plug-and-play device that simply plugs into a USB (Universal Serial Bus) port of a computer. The computer detects it automatically as removable drive.

pen drive memory

Pen drive is a compact device which resemble the shape of pen that can be used to quickly transfer audio, video and data files from hard drive of one computer to another.

Nowadays it came in various shapes and various added features like camera or built in radio player.

It is compatible with most PC’s and is ideal external storage for mobile people to carry or transfer data from one computer to other.

Nowadays the pen derives are available in different storage capacities form megabytes to gigabytes.

FAQ(Frequently Asked Questions)

Q1. What are Memory Concepts?

Ans. An essential feature of every computer is the memory. Memory is used for storing both instructions to be executed in the computer and the result to be stored for future use.

Memory units are the internal storage area in the computer. The term “memory” identifies data storage that comes in the form of chips and the word “storage” is used for memory that exists on tapes or disks.

Moreover, the term memory is usually refers to the actual chips capable of holding data. Every computer comes with a certain amount of physical memory usually referred to as the main memory or the RAM.

You can think of the main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory therefore can, hold about 1 million bytes of information.

Q2. What is Bit?

Ans. It is the space required to store one bit (0 or l) in the memory.

Q3. What is Nibble?

Ans. Collection of 4 bits is known as nibble.

Q4. What is Byte?

Ans. Collection of 8 bits is known as byte.

  • 1 Kilobyte (1 KB): 1024 bytes.
  • 1 Mega Byte (1 MB): 1024 KB= 1024 X 1024 bytes.
  • Giga Byte (1 GB): 1024 MB=1024 x 1024 x 1024 bytes.
  • Tera Byte (1 TB): 1024 GB =1024 X 1024 X 1024 X 1024 bytes.
  • Peta Byte (I PB): 1024 113-1024 X 1024 X 1024 x 1024 x 10/4 bytes

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