Secondary Memory And Secondary Storage Devices




There are four main types of secondary storage devices available in a computer system:

  1. Disk drives
  2. CD drives (CD-R, CD-RW, and DVD)
  3. Tape drives
  4. USB flash drives
  • Hard disk, floppy disk, compact disk (CD), Digital Versatile Disk (DVD) and magnetic tapes are the most common secondary storage mediums.
  • Hard disks provide much faster performance and have larger capacity, but are normally not removable; that is, a single hard disk is permanently attached to a disk drive.
  • Floppy disks, on the other hand, are removable, but their performance is far slower and their capacity far smaller than those of hard disks.
  • A CD-ROM or DVD -ROM is another portable secondary memory device. CD stands for Compact Disc. It is called ROM because information is stored permanently when the CD is created. Devices for operating storage mediums are known as drives.

Most of the drives used for secondary memory are based on electro-mechanical technology. Mechanical components move much more slowly than do electrical signals. That’s why access to secondary memory is much slower than access to main memory.

Floppy Disk

The floppy disk is a thin, round piece of plastic material, coated with a magnetic medium on which information is magnetically recorded, just as music is recorded on the surface of plastic cassette tapes. The flexible floppy disk is enclosed inside a sturdier, plastic jacket to protect it from damage. The disks used in personal computers are usually 31⁄2 inches in diameter and can store 1.44 MB of data. Earlier PCs sometimes used 51⁄4 inch disks. The disks store information and can be used to exchange information between computers. The floppy disk drive stores data on and retrieves it from the magnetic material of the disk, which is in the form of a disk. It has two motors one that rotates the disk media and the other that moves two read-write heads, each on either surface of the disk, forward Floppy Disk Drive or backward.

Hard Disk

A hard disk is a permanent memory device mounted inside the system unit. Physically, a hard disk consists of one or more metal (sometimes aluminum) platters, coated with a metal oxide that can be magnetized. The platters are all mounted on a spindle, which allows them to spin at a constant rate. Read/write heads are attached to metal arms and positioned over each of the platter surfaces. The arms can move the read/write heads radially inwards and outwards over the surfaces of the platters. Data and programs are stored on the hard disk by causing the write heads to make magnetic marks on the surfaces of the platters. Read heads retrieve the data by sensing the magnetic marks on the platters. The surface of each platter is divided into concentric rings called tracks. The tracks form concentric circles on the platter’s surface. Each track is divided into a certain number of sectors. A sector is capable of generally 512 bytes or sometimes 1,024 bytes of data. The head is mounted on an arm, which moves or seeks from track to track. The vertical group of tracks at the same position on each surface of each platter is called a cylinder. Cylinders are important, because all heads move at the same time. Once the heads arrive at a particular track position, all the sectors on the tracks that form a cylinder can be read without further arm motion. The storage capacity of a hard disk is very large and expressed in terms of gigabytes (GB). The data that is stored on the hard disk remains there until it is erased or deleted by the user. The hard disk drive provides better performance and become mandatory for computer systems for the following reasons:

  1. Higher capacity of data storage
  2. Faster access time of data
  3. Higher data transfer rates
  4. Better reliability of operation
  5. Less data errors or data loss

CD – Compact Disk

A CD is a portable secondary storage medium. Various types of CDs are available: CD-R and CD-RW. CD-RW drives are used to create and read both CD-R and CD-RW discs. Once created (i.e. when it has been “burned”), data stored on CD-R (CD- Recordable) disc can’t be changed. On the other hand, a CD- Rewritable (CD-RW) disc can be erased and reused.

This disk is made of synthetic resin that is coated with a reflective material, usually aluminum. When information is written by a CD-writer drive, some microscopic pits are created on the surface of the CD. The information bit on a CD-ROM surface is coded in the form of ups and downs (known as pits and dumps), created by infrared heat. There is one laser diode on the reading head. The bits are read by shining a low - intensity laser beam onto the spinning disc. The laser beam reflects strongly from a smooth area on the disc but weakly from a pitted area. A sensor receiving the reflection determines whether each bit is a 1 or a 0 accordingly.

CDs were initially a popular storage media for music; they were later used as general computer storage media. Most personal computers are equipped with a CD-Recordable (CD-R) drive. A CD-Rewritable (CD-RW) disc can be reused because the pits and flat surfaces of a normal CD are simulated on a CD- RW by coating the surface of the disc with a material that, when heated to one temperature becomes amorphous (and therefore non-reflective) and when heated to a different temperature becomes crystalline (and therefore reflective).



Frequently Asked Questions

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Ans: All modern computers use semiconductor memory as primary memory. One of the important semiconductor memories used in desktop computers is Random Access Memory (RAM). Here “random access” means that any storage location can be accessed (both read and write) directly. view more..
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Ans: The abbreviated form of “ bi-nary dig-it ” is known as bit. When a “bit” is mentioned, it means a single binary digit, which may either be a “0” or “1”, is referred to. view more..
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Ans: A binary number is made of 0s and 1s. In the binary number system only two symbols, 0 and 1, are used to represent numeric values. The symbol “0“ represents the value “zero” while the symbol “1” represents the value “one”. Since there are only two symbols in the binary number system, the value “two” is represented by placing the symbol “1” on the left–hand side of the symbol “0” resulting in the binary equivalent “10”. Next, the value “three” is represented by “11” in the binary number systemby replacing the “ 0 ” in “10” by the next higher value symbol “1”. view more..
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Ans: Secondary Memory And Secondary Storage Devices view more..
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Ans: There are four main types of secondary storage devices available in a computer system: Disk drives CD drives (CD-R, CD-RW, and DVD) Tape drives USB flash drives view more..
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Ans: Cache memory It is a special high-speed memory that allows a microprocessor to access data more rapidly than from memory located elsewhere on the system board. view more..
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Ans: A microprocessor is an integrated circuit chip that contains all of the essential components for the central processing unit (CPU) of a microcomputer system. view more..
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Ans: 1’s complement A number system that was used in some computers to represent negative numbers. To form 1s complement of a number, each bit of the number is inverted which means zeros are replaced with ones and ones with zero. view more..
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Ans: 1’s complement A number system that was used in some computers to represent negative numbers. To form 1s complement of a number, each bit of the number is inverted which means zeros are replaced with ones and ones with zero. view more..
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Ans: 2’s complement A number formed by adding 1 to the 1’s compliment of a number. The 2’s complement representation has become the standard method of storing signed binary integers. It allows the representation of an n-bit number in the range – 2n to 2n-1, and has the significant advantage of only having one encoding for 0. view more..
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Ans: A Boolean or logic expression is a logic variable or a number of logic variables involved with one another through the logical operations ‘.’, ‘+’, and ‘–’. view more..
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Ans: The AND gate is an electronic circuit that has two or more inputs and only one output. It gives a HIGH output (1) only if all its inputs are HIGH. view more..
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Ans: The OR gate is an electronic circuit that has two or more inputs and only one output. It gives a HIGH output if one or more of its inputs are HIGH. view more..
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Ans: NOT GATE also called INVERTER.The inverter is a little different from AND and OR gates as it has only one input and one output. Whatever logic state is applied to the input, the opposite state will appear at the output. The NOT function is denoted by a horizontal bar over the value to be inverted. view more..
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Ans: The Exclusive-OR or XOR gate is a two-input circuit that will give a HIGH output if either, but not both, of the inputs are HIGH. The XOR function is an interesting and useful variation of the basic OR function. Its function can be stated as ‘Either A or B, but not both’. The XOR gate produces a logic 1 output only if the two inputs are different. If the inputs are the same, the output is a logic 0. XOR is also called an anti-coincidence gate or inequality detector. view more..
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Ans: The Exclusive-NOR gate is a XOR gate followed by a NOT gate. XNOR gate is a two-input and one-output logic gate circuit. In the gate, the output is HIGH if both inputs are either LOW or HIGH view more..
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Ans: A programming language can be defined formally as an artificial formalism in which algorithms can be expressed. It is composed of a set of instructions in a language understandable to the programmer and recognizable by a computer. Computer languages have been continuing to grow and evolve since the 1940’s. Assembly language was the normal choice for writing system software like operating systems, etc. But, C has been used to develop system software since its emergence. The UNIX operating system and its descendants are mostly written in C. Application programs are designed for specific computer applications. Most programming languages are designed to be good for one category of applications but not necessarily for the other. For an instance, COBOL is more suitable for business applications whereas FORTRAN is more suitable for scientific applications. view more..
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Ans: A program, written in source language, is translated by the compiler to produce a program in a target language. The source language is usually a high-level language. The target language may or not necessarily be machine language. In most cases, the target language is assembly language, and in which case, the target program must be translated by an assembler into an object program. Then the object program is linked with other object programs to build an executable program, which is normally saved in a specified location of the secondary memory. When it is needed to be executed, the executable file is loaded into main memory before its execution. The whole process is managed, coordinated and controlled by the underlying operating system. Sometimes the target language may be a language other than machine or assembly language, in which case a translator for that language must be used to obtain an executable object program. view more..




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