Monday, April 9, 2007

Topic 3 : Hardware

All Components Of A Computer

All computers, whatever their size or function, have certain basic components. They have input devices for reading data into main memory, a central processing unit (CPU) for processing the data, output devices for printing, displaying or ouputting information, and auxiliary storage devices for permanent storage of programs and data.


The Processor

The processor has the following functions:
--> fetches the next instruction;
--> decodes the instruction,
--> executes the decoded instruction.

Most computers use integrated circuits, or chips, for their processors and main memory. A chip is about 1cm square and can hold millions of electronic components such as transistors, and resistors. The CPU of a microcomputer is called as a microprocessor. The processor and main memory of a PC are commonly held on a single board called a motherboard.

Main Memory

The program currently being executed and the data used by this program are held in main memory, which is divided into millions of individually addressable storage units called bytes. One byte can hold one character, or it can be used to hold a code representing, for example, a tiny part of a picture, a sound, or part of a computer program instruction. The total number of bytes in main memory is reffered to as the computer's memory size.

Computer mamory sizes are measured as follows:

1 Kilobyte (Kb) = 1024 bytes
1 Megabyte (Mb) = 1024 Kb = 1, 048, 576 bytes (about 1 million)
1 Gigabyte (Gb) = 1024 Mb = 1, 073,741,842 bytes (about 1 billion)
1 Terabyte (Tb) = 1024 Gb = 1, 099,511,627,776 bytes (about 1 triillion)

RAM and ROM

There are basically two kinds of main memory: Random Access Memory (RAM) which is the ordinary kind of main memory referred to above, used for storing programs which are currently running and data which is being processed. This type of memory is volatile which means that it loses all its contents as soon as the machine is switched off.

Read Only Memory (ROM) is the other type of main memory, and this is non-volatile, with its contents permanently etched into the memory chip at the manufacturing stage. It is used for example to hold the bootstrap loader, the program which runs as soon as the computer is switched on and instructs it to load the operating system from disk into main memory (RAM). It may also store fixed data associated with the computer system. In special purpose computers used in video recorders, washing machines and cars, the program instructions are stored in ROM.

Cache Memory

Cache memory is a type of very fast memory that is used to improve the speed of a computer, doubling it some cases. It acts as an intermediate store between the CPU and main memory, and works by storing the most frequently or recently used intructions and data so that it will be very fast to retrieve them again. Thus when an item of data is required, a whole block of data will be read into cache in the expectation that the next piece of data required is likely to be in the same block. The amount of cache memory is generally between 1Kb and 512Kb or more.

Disk Storage

The most common form of auxiliary storage (also known as external or secondary memory or backing store) is disk. All standalone PCs come equipped with an in-built hard disk, the capacity of which is also measured in bytes. A typical hard disk for a PC stores gigabytes, and is used for storing software including the operating system, other systems software, application programs and data for long term storage.

Floppy disks consist of a thin sheet of mylar plasticencased in a hard 3 1/2 "casing. The standard type of disk in use today has a capacity of 1.44Mb. Flash memory cards or sticks are rapidly replacing floppy disks. These can hold from 32Mb to 1Gb.




Primary and Secondary Storage

A computer's main memory (RAM) is known as primary storage. In order to execute a program, the program instructions and the data on which it is to operate .have to be loaded into main memory. Primary storage, however is volatile; when the computer is switched off, all the contents of memory are lost. This is one good reason to perform frequent saves to disk when working on, for example, a word processed document.

A more permanent, non-volatile form of storage is required by all computer systems to save software and data files. Magnetic tape, magnetic disks, CD-ROM (Compact Disk Read Only Memory), and microfilm are all examples of what is known as secondary storage.

Magnetic Disk Storage

A magnetic disk consists of two surfaces, each of which contains concentric circles called tracks. Each track is divided into sectors. If you reformat a disk that already has data on it, all the data will be erased (although you can also do a 'quick format' which erases only the file directory).

Floppy Disks



The
standard 3 1/2 “floppy disk is a thin, flexible plastic disk coated in metal oxide, enclosed in a rigid plastic casing. A standard high density disk has a storage capacity of 1.44 Megabytes. The disk can be removed from the drive unit and is highly portable. Floppy disks are inexpensive but easily damaged.



Hard Disks For Microcomputers

The hard disk used with PCs consists of one or more disk platters permanently sealed inside a casing. Hard disks typically have a capacity of between 40Gb and 160Gb or more.

External hard drives, which can be plugged into a microcomputer, are available as extra storage.

Hard Disks For Minis And Mainframes

For large-scale applications storing huge amounts of data, several hard disk units will be required. The disks may be either fixed {sealed inside the unit) or removable. Fixed disks are faster, more reliable, and have a greater storage capacity.

As with other types of disk, data is stored on concentric tracks, with tracks being divided into sectors. All the tracks that are accessible from one position of the read-write heads form a cylinder; data is recorded cylinder by cylinder to minimise movement of the read-write heads, thereby minimising access time.

Optical Disks

Random access time is longer than that of a hard disk, but some optical storage is writeable or even re- writeable, CD-ROM, DVD-ROM (Read-only), CD-R, DVD-R (Writeable), CD-RW, DVD-RW (Re- Writeable).

CD-ROM

CD-ROMs can store around 680Mb of data, equivalent to hundreds of floppy disks. The data may be in text form, or may be in the form of graphics, photographic images, video clips or sound files. Although they do not transfer data as fast as a hard disk drive, their speed in increasing every year and is acceptable for most applications.

As the name suggests, the disks are read-only. When the master disk is created, a laser beam burns tiny pits in the surface of the disk, which (unlike a magnetic disk) has a single spiral track divided into sectors. To read data from the disk, a laser beam is reflected off the surface of the disk, detecting the presence or absence of pits which represent binary digits. CD- ROMs are widely used for distribution of software, multimedia files, catalogues and technical manuals.

CD-R Disks

Write Once, Read Many optical laser disks (WORM disks) look similar to CD-ROM disks, but they are often gold rather than silver in colours. An end-user company can use these disks to write their own material, typically for archiving or storing, say, graphic or photographic images which will not be changed.

CD-RW Disks

These are re-writeable disks and are more expensive than CD-R. These can be used for backing up where they may need to be overwritten.

DVD-ROM

Digital Versatile Disk Read-Only Memory. These disks are the same size as CD-ROMs and are made using similar materials and manufacturing techniques. They store about seven times as much data as a CD- ROM, because the track spacing and pit dimensions are smaller.

Flash Memory

Flash is electrically erasable programmable read-only memory (EEPROM). It is used in memory cards for peripheral devices such as digital cameras, mobile telephones, PDAs and MP3 players. It is also available as USB memory sticks and, when plugged into a computer's USB port, behaves like an external disk drive. Flash memory is inexpensive, high-capacity storage and is rapidly replacing floppy disks as portable secondary storage.

Thursday, April 5, 2007

Hardware


Magnetic Tape

Magnetic tape
is a non-volatile sequential storage medium consisting of a magnetic coating on a thin plastic strip. It is used for data collection, backup and archiving. Nearly all recording tape is of this type, whether used for video, audio storage or general purpose digital data storage using a computer.


Tape has been more economical than disks for archival data, but that is changing as disk capacities have increased enormously. If tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other.

Sequential Medium

A major drawback of tape is its sequential format. Locating a specific record requires reading every record in front of it or searching for markers that identify predefined partitions. Although most tapes are used for archiving rather than routine updating, some drives allow rewriting in place if the byte count does not change. Otherwise, updating requires copying files from the original tape to a blank tape (scratch tape) and adding the new data in between.

Uses of magnetic tape

Video recording




Video recording demands much higher bandwidth than audio recording and was made practical by the invention of helical scan. Early video recorders were reel-to-reel but modern systems use cartridge tapes and videocassette recorders are very common in homes and television production facilities, though many functions of the VCR are being replaced by optical disc media.


Data storage

The use of magnetic tape for data storage has
been one of the constants of the computer industry.

Tape has quite long data latency for random accesses since the deck must wind an average of ⅓ the tape length to move from one arbitrary data block to another. Most tape systems attempt to alleviate the intrinsic long latency, either using indexing, where a separate lookup table is maintained which gives the physical tape location for a given data block number, or by marking blocks with a tape mark that can be
detected while winding the tape at high speed.

Tape remains a viable alternative to disk due to its lower cost per bit. Tape has historically offered enough advantage in cost over disk storage to make it a viable product, particularly for backup, where media removability is also important. The rapid improvement in disk storage density and price, coupled with arguably less-vigorous innovation in tape storage, has reduced the market share of tape storage products.

Information is from http://en.wikipedia.org/wiki/Magnetic_tape
Pictures are searched through google