Storage Devices

Storage Devices

Those devices that store the data on the computer (temporary or permanent) are called as “Storage Devices”.

TYPES/ DIVISION

There are basically two types of storage devices:

_ Main Memory (Primary Storage Devices)

_ Secondary Memory (Secondary Storage Devices)

MAIN MEMORY

It is also called as the primary memory of the computer. It is made up of semi-conductor material.

ADVANTAGES

_ It is very fast.

_ It is directly accessible by the CPU.

_ It has low cost.

FUNCTIONS

_ Main memory accepts data from the input unit.

_ Exchanges them with other parts of the CPU for processing.

_ It stores the instructions.

TYPES

RAM (Random Access Memory)

ROM (Read Only Memory)

RANDOM ACCESS MEMORY

It is also called as volatile memory (temporary storage) which means that any data in RAM shall be erased as soon as the power is cut off. It is a very crucial memory and plays vital role in the operation of the computer system. All the instructions being fed to the CPU are first being copied in to RAM. It can perform both read and write operations.

READ OPERATION

The process of copying data from memory is called as READ.

WRITE OPERATION

The process of writing data onto the memory is called as WRITE.

RAM can be divided in to two main types:

_ DRAM

_ SRAM

READ ONLY MEMORY (ROM)

It is a non-volatile memory, so any thing that is written on ROM is permanent in nature. ROM contains set of instructions which help computer at its startup. As when power of the computer is turned on RAM is empty, thus computer does not know what to do at this time. This is when ROM plays its role and copies a pre-defined set of instructions into the RAM so that the computer can read in and act upon these instructions for proper working. On ROM only READ operation can be performed.

ROM can be divided in to three main types:

_ PROM (Programmable Read Only Memory)

_ EPROM (Erasable PROM)

_ EEPROM (Electrically EPROM)

SECONDARY MEMORY (BACKING STORAGE)

Those devices that can store the data permanently are called as “Secondary Devices”.They are also known as External/Auxiliary Storage and backing storage devices. Secondary storage provides backup to primary memory for further utilization, that’s why it is also called as backing storage devices.

EXAMPLE

Hard disks, floppies, CDs etc are secondary storage devices.

PURPOSES

_ Provide long-term storage data/information.

_ Give additional memory to computer to save data and to provide backup to primary memory.

_ Increase the speed of accessing data, which is not possible while using primary memory i.e. volatile in nature.

_ Provide security to data in case of switching off computer i.e. non-volatile.

TYPES

There are three types of the secondary storage devices.

_ Magnetic Tape

_ Magnetic Disks (Hard disks and diskettes)

_ Optical Disks

MAGNETIC TAPE

The surface of tape is coasted white iron oxide, which can store data in compressed form. It can carry 9 rows of magnetic spots. Each row is called as a “track” having its own reading/writing head for playing or recording the information.

It is generally used for making backup copies of data which is stored in hard disk.

MAGNETIC DISKS

They are found in two types:

_ Interchangeable Magnetic Disks

_ Fixed Magnetic Disks/ Hard Disks

INTERCHANGEABLE MAGNETIC DISKS

_ Floppy disks are the most common and popular types of secondary storage.

_ Floppy disks are flexible.

_ The data on it can be accessed randomly.

_ On the diskette data is stored in rings called as “tracks”. Each track is divided into eight or nine “Sectors”.

_ Sectors are that sections on which data is stored. The spacing of the tracks is measured in tracks per inch (TPI).

HARD DISK OR FIXED MAGNETIC DISK

Hard disk is rigid in nature and usually made up of aluminum with asurface coating of easily magnetized elements such as iron, chromiumand nickel.

OBJECTIVES OF MANUFACTURING HARD DISKS

There are three main objectives of hard disk manufacturers:

_ To store more data/information in less space.

_ To enable a more rapid transfer of information to/from RAM.

_ Providing a much reliable storage medium for data.

MEMORY UNITS

The capacity of the computer storage is expressed in number of bits, bytes or characters.

BITS

The basic memory unit of a digital computer is a bit, which stands for binary digit. Bit represents a binary number that is either 0 or 1.The memory of a computer is composed of cells. Each cell contains on bit of information. Each memory cell has a value of 0 or 1 by ON andOFF state of the cell.

BYTE

A byte is a basic unit of measurement of information storage in computer. It is an ordered collection of bits, with each bit denoting a single binary value of 1 or 0. The byte most often consists of 8 bits in modern systems; however, the size of a byte can vary and is generally determined by the underlying computer operating system or hardware.

THE WORD

In computers, data/instructions flow in groups of bits called Words.

Word size is the number of bits a microprocessor handles at a time.This means 8-bit microprocessor can manipulate 8 bits of data at a time.

SIMMs (SINGLE IN LINE MEMORY MODULES)

Many RAM chips are installed on a small circuit board. These RAM chips are wired together to form a single module of large memory called SIMMs. SIMMs have different capacities such as 1 megabyte, 4 megabytes, 16 megabytes etc.

DIMMs (DUAL IN LINE MEMORY MODULES)

DIMMs have large storage capacity than the SIMMs. DIMMs have more speed (rate of data transfer) that the SIMMs.

TYPE OF DATA ACCESS METHODS

There are basically two types of data access methods:

_ Direct/Random Access

_ Sequential Access

DIRECT/RANDOM ACCESS

_ Access the data on the device directly.

_ Don’t require reading from the beginning of the file to find a particular record.

_ Direct access is addressable.

_ It is faster.

EXAMPLES

_ Magnetic Disks

_ Compact Disks (CD)

SEQUENTIAL ACCESS

_ Access the data on the device sequentially.

_ It requires reading from the beginning of the file to find a particular record.

_ It is slower (time consuming).

EXAMPLES

_ Magnetic Tapes

_ Audio & Video Cassettes