COMPUTER MEMORY

Computer Memory

The computer memory is one of the most important elements in a computer system. It is the internal or external storage area, which holds the data and instructions during processing in the form of binary numbers.

It also relates to many devices and components that are responsible for storing data and applications on a temporary or a permanent basis.

Computer memory can be classified into two types:

(i) Primary memory and

(ii) Secondary memory/Storage device.

Primary Memory

It is also known as Main Memory. It is the internal storage memory used by computer to hold data and instructions. The primary memory has limited storage capacity.

Primary memory holds the data and programs needed at that instant by CPU.

Primary memory is volatile in nature i.e. it requires constant power supply to motion the current information.

There are two types of Primary Memory:

(i) RAM

(ii) ROM

(i) RAM

RAM stands for Random Access Memory. It is the internal memory that can be read from as well as written to. This memory is often associated with volatile types of memory. It can hold data only on temporary basis because it requires a continuous flow of electrical current.

If current is interrupted, data is lost. It is an integrated circuit that enables you to access the stored data in a random order constantly.

The two main forms of RAM are:

(a) Static RAM It is also written as SRAM. It is a computer memory that requires a constant power flow in order to hold information. SRAM is more expensive and requires more power therefore it is commonly used in cache and video card memory. It is faster than DRAM.

(b) Dynamic RAM It is also written as DRAM. It stores information in a cell containing a capacitor and transistor, these cells must be refreshed with electric impulses in few milliseconds. This process allows memory to keep charge and hold the data as long as needed.

There are some enhanced versions of RAM, which are as follows:

(a) EDORAM (Extended Data Output Random Access Memory)

(b) SDRAM (Synchronous Dynamic Random Access Memory)

(c) DDR SDRAM (Double Data Rate Synchronous Dynamic Random Access Memory)

(ii) ROM

In ROM (Read Only Memory) memory, information once stored remain fixed, i.e. it cannot be changed. So, ROM can only be read and used.

Generally, it contains a set of start-up instructions, i.e. what to do when a computer is turned ON. The contents of ROM remain stored even if power is turned OFF. This memory is often associated with non-volatile types of memory. A ROM cannot be altered once the chip has been made.

ROM is further sub divided into several types:

(a) PROM (Programmable Read Only Memory) It is a computer memory chip capable of being programmed after it has been created. But once the PROM has been programmed, the information written is permanent and cannot be erased or deleted.

(b) EPROM (Erasable Programmable Read Only Memory) It is a computer memory chip on which the written information can be changed by exposing to ultra voilet light. It is just like a small glass circle that expose the chip that can be re-programmed.

(c) EEPROM (Electrically Erasable Programmable Read Only Memory) It is a PROM that can be erased and reprogrammed using an electrical charge.

Cache Memory

Cache (pronounced cash) memory is extremely fast memory that is built into a computer’s central processing unit, or located next to it on a separate chip. The CPU uses cache memory to store instructions that are repeatedly required to run programs, improving overall system speed.

As the microprocessor processes data, it first looks in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the time-consuming reading of data from larger memory.

The advantage of cache memory is that if needed data is found in cache then the CPU does not have to use the motherboard’s system bus for data transfer and thus processing gets fast.

Storage Devices/Secondary Memory

Secondary memory, also known as Secondary Storage or Auxiliary Memory, is the slower and cheaper form of memory. It is a permanent storage device.

CPU does not access the secondary memory directly. The content in it must first be copied into the RAM to be processed. Secondary memory is non-volatile in nature. i.e. the information does not get erased even when power is switched OFF and data will not be destructed until and unless the user erases it.

Secondary Memory devices include

Magnetic disksOptical discsSolid State
Floppy DiskCDPen/Flash Drive
Hard Disk DriveDVD 
Magnetic TapeBlue-ray Disc 

(i) Floppy Disk (Diskette)

A Floppy Disk Drive (FDD) is a computer disk drive that enables a user to save data on removable diskettes. This portable storage device is a rewritable media and can be reused a number of times. It is made of plastic with magnetic coating on it. It is round in shape and is covered by square plastic jacket.

Floppy disks are commonly used to move files between different computers. Although 8″ disk drives made available in 1971 were the first real disk drives, the first widely used floppy disk drives were the 4″, 5 ” floppy disk drives, which were later replaced with 3” floppy disk drives.

However, these drives were replaced with CD-R and other writeable disc drives and flash drives.

5 ” diameter-floppy has a capacity of 1.2 MB.

3 ” diameter-floppy has a capacity of 1.44 MB.

(ii) Hard Disk Drive

A Hard Disk Drive (HDD) is a non-volatile, random access digital data storage device. It is a data storage device used for storing and retrieving. Digital information using rotating disks (platters) coated with magnetic material. All programs of a computer are installed in hard disk within a particular drive.

It consists of a spindle that hold non-magnetic flat circular disks called platters which hold the recorded data. Each platter requires two read/write heads that are used to write and read the information from a platter. All the read/write heads are attached to a single access arm so that they cannot move independently.

The information is recorded in bands where each band of information is called a track.Each platter has the same number of tracks and a track location that cuts across all platters is called a cylinder. The tracks are divided into pie-shaped sections known as sectors.

(iii) Magnetic Tape

It is a medium of data storage which is made up of a magnetic material. It is a storage medium on a large open reel or in a smaller cartridge or cassette (like a music cassette). It is used for the applications that are based on sequential data processing i.e. it is a sequential data access medium.

Due to this (sequential data access) nature, these tapes are not suitable for data files that need to be revised or updated often. They are generally used to store backup data or that type of data, which is not frequently used or to transfer data from one system to another.

(iv) Compact Disc (CD)

CD is an optical media that is used to store digital data.

The compact discs are relatively cheap storage devices.

Compact Disc (CD) are categorized into three main types:

(a) CD-ROM (Compact Disc-Read Only Memory) Data is recorded permanently on the surface of the optical disk through the use of laser.

The recorded content cannot be changed or erased by users. It is also called WORM (Write Once Read Many) disc. It is capable of storing large amounts of data- up to 1GB, although the most common storage capacity is 700 MB.

(b) CD-R (Computer Disc- Recordable) Data can be written on these discs only once. The data once stored in these discs cannot be erased.

(c) CD-RW (Compact Disc- Rewritable) It is an erasable disc. CD-RW is used to write data multiple times on a disc by the use of format feature.

(v) Digital Video Disc (DVD)

Digital Video Disc (DVD) is also known as Super Density (SD) Disc. A DVD is an optical disc storage media manufactured for the first time by Philips, Sony, Toshiba and Panasonic in 1995. DVDs offer higher storage capacity than DVD compact discs while having the same dimensions.

Depending upon the disk type, DVD can store several Gigabytes of data (4.7 GB-17.08GB).

DVDs are primarily used to store music or movies and can be played back on your television or on the computer too. They are not rewritable media.

DVDs come in three varieties:

(i) DVD ROM (Digital Video Disc-Read only Memory)

(ii) DVD-R (DVD-Recordable)

(iii) DVD-RW (DVD-Rewritable)

(vi) Blu-ray Disc

Blu-ray Disc (official abbreviation BD) is an optical disc storage medium designed to recapture the data normally in DVD format. Blue-ray disc contain 25 Gb (23.31 GB) per layer space. The name blu-ray disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density.

Blu-ray can hold almost 5 times more data than a single layer DVD.

The variations in the formats are as follows:

(a) BD-ROM (Read only) for pre-recorded content

(b) BD-R (Recordable) for PC data storage

(c) BD-RW (Rewritable) for PC data storage

(d) BD-RE (Rewritable) for HDTV recording

(vii) Pen/Flash/Thumb Drive

A flash drive is a data storage device that consists of flash memory (USB memory/key memory) with an integrated, Universal Serial Bus (USB) interface. USB flash drives are typically removable, rewritable and physically much smaller than a floppy disk. A USB flash drive, data stick, thumb drive and a pen drive is a portable drive that is same as the size of your thumb that connects to the computer USB port.

Today, flash drives are available in various storage capacities as 256MB, 512MB, 1GB, 4GB, 16GB and 64 GB etc. Flash drives are widely used as an easy and small medium to transfer and store the information from the computers.

Basic Units of Measurement

When User use a RAM, ROM, Floppy disk or Hard disk, the data is measured using some units. In computer terminology, they are called Nibble, Bit, Byte, Kilobyte and Gigabyte etc.

Units of computer memory measurements

Bit (Binary digit) The smallest unit of data. It is either 0 or 1.

Nibble A group of 4 bits or half a byte.

Byte A group of 8 bits. A byte can represent 256 (28) distinct values, such as the integers from 0 to 255. Each keyboard character is represented through atleast 1 byte.

Kilobyte (KB) It is actually 210 bytes or 1 KB= 1024 bytes.

Megabyte (MB) It is actually 220 bytes or 1 MB = 1024 KB.

Gigabyte (GB) It is actually 230 bytes or 1 GB = 1024 MB.

Terabyte (TB) It is actually 240 bytes or 1 TB = 1024 GB.

Petabyte (PB) It is actually 250 bytes or 1 PB= 1024 TB.

Exabyte (EB) It is actually 260 bytes or 1 EB= 1024 PB.

Zettabyte (ZB) It is actually 270 bytes or 1 ZB = 1024 EB.

Yottabyte (YB) It is actually 280 bytes or 1 YB = 1 24 ZB.

Brontobyte It is actually 290 bytes or 1 Brontobyte = 1024 YB

Geopbyte It is actually 2100 bytes or 1 Geopbyte = 1024 Brontobyte.

Geopbyte is the highest memory measurement unit.

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