Computer System Concept

Computer System Concept

Computer system is made of  major components: Hardware and Software.

  • The computer hardware is the physical equipment.
  • The software is the collection of programs (instructions) that allow the hand ware to do its job.  Below figure represents a computer system.


The hardware component of the computer system contains five parts:  

(1) Input devices    (2) Central processing unit (CPU)    (3) primary storage, or main memory

(4) output devices and  (5)Auxiliary storage devices (Figure 1-2).

The brain of the computer is the Central Processing Unit (CPU), represented by a single chip on a PC. The CPU carries out every instruction stored in a program, while interacting with other agencies as and when necessary. Most of the work is done by the Arithmetic and Logic Unit (ALU), which is an integral part of the CPU.

The CPU needs both fast and slow memory to work with. Fast memory is represented by primary memory, also known as Random Access Memory (RAM). It is divided into a number of contiguously numbered cells. The numbers represent the addresses of the cells. The CPU accesses a memory location by sending its address to the memory unit. Primary memory is used for storing instructions and data of the program currently in execution (online storage).

The computer also supports slower secondary memory, also called secondary storage or auxiliary memory. This is generally the hard disk but it can also be a CD-ROM or DVD-ROM. Secondary memory is used for storing data not required currently (offline storage). Data in secondary memory are stored as files having unique names. A program is executed by loading instructions and data from secondary memory to primary memory.


The CPU has evolved from a bulky vacuum tube-based unit of the 1940s to a modern 5cm square chip that is commonly called the microprocessor, or simply, processor. It comprises the following components:

  • Arithmetic and Logic Unit (ALU)
  •  Control Unit (CU)
  • Special-purpose registers
  • A clock

Acting in tandem, they control and perform all arithmetic and logical operations. This chip generates a lot of heat, the reason why it is mounted on the motherboard with both a heat sink and a fan to dissipate the heat.

The central processing unit (CPU) is responsible for executing instructions such as arithmetic calculations, comparisons among data, and movement of data inside the system.

The Arithmetic and Logic Unit (ALU) is a “super calculator”. Apart from carrying out all arithmetic tasks, the ALU compares two numbers and performs boolean operations (like AND, OR and NOT) on them. The Control Unit (CU) doesn’t perform any computational tasks. Using signals, the CU controls the way data is moved between the various components of the computer. Both ALU and CU use the services of a clock for sequencing and synchronizing their operations.

(3) Primary storage, or main memory

Primary storage, also known as main memory, is a place where the programs and data are stored temporarily during processing. The data in primary storage are erased when you turn off a personal computer or when you log off from a time-sharing computer.

Memory is an essential resource used by the computer and its users. The CPU needs memory for the currently running program. Users need memory to back up programs and data that are not needed currently. For meeting the differing requirements, the computer supports multiple types of memory. For convenience, we categorize memory into two types-primary and secondary This section examines primary memory which includes the following types:

  •  Random Access Memory (RAM-SRAM and DRAM)
  • Read Only Memory (ROM, PROM,EPROM, EEPROM) .
  • Cache Memory (L1, L2 and L3)
  • CPU Registers


The computer also supports secondary memory, also known as secondary storage or auxiliary memory. This memory type is not directly connected to the CPU but it can exist both in the machine or external to it. Secondary memory is non-volatile and thus meets the requirements of offline and long-term storage. These devices are slower than primary memory but they have larger capacities. They are also way cheaper than primary memory; GB of hard disk space is much cheaper than I GB of RAM.

The last couple of decades have seen the emergence of multiple types of storage devices. They have different physical sizes and capacities and may or may not require power. In this section, we consider the following storage devices with their typical capacities indicated in parentheses:

  • Hard disk including the portable disk (500 GB to 4 TB).
  • Magnetic tape (20 TB).
  • CD-ROM (700 MB-less than 1 GB).
  • DVD-ROM (4.7 GB and 8.5 GB).
  • Blu-ray disk (27 GB and 50 GB).
  • Flash memory based on the EEPROM (1 GB to 128 GB).
  • The obsoleted floppy disk (1.2 MB and 1.44 MB).

The Hard Disk

Also known as hard drive or fixed disk because it is fixed inside the computer, the hard disk represents one of the oldest but most commonly used forms of secondary storage. Hard disks have very high capacities with the cheapest cost. It is common for a laptop to have 500 GB of storage. Desktops
 typically have 1 TB (terabyte) of storage. The hard disk is no longer an offline device because it houses the computer’s operating system along with all programs and data.

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hard disk

Magnetic Tape

The age-old magnetic tape is still around thanks to the enhancements that have been made to this device. The basic technology has not changed though; the tape is made of a plastic film with one side coated with magnetic material. The entire mechanism comprising two spools and the tape is encapsulated in a small cassette or cartridge. Current technology supports capacities of 1 TB or more, but 200 TB tapes are expected to be launched in the near future. The device is not fully portable though because a separate tape drive is required, and most computers don’t have one.

Data are read from and written to the tape using a read-write head and an erasure head. The write operation is preceded by the erasing operation.

Optical Disks: The CD-ROM, DVD-ROM and Blu-Ray Disk

Non-volatile read-only memory, which we saw in the ROM family (including PROM, EPROM and EEPROM), is also available on optical disks. These disks, comprising mainly the CD-ROM and DVD-ROM, can hold large volumes of data (700 MB to 8.5 GB) on inexpensive media. The Blu-ray disk enhances this capacity to 50 GB. A laser beam in their drives controls the read and write operations. Because most PCs and laptops have these drives (not Blu-ray though), data backups on optical disks are easily distributed.

Optical disks are made of carbonate material with a thin layer or two of reflective film. A laser beam is used to construct pits and lands by burning (writing) selected areas along its tracks. When reading data, the beam is reflected from a land (signifying a 1) but not from a pit (signifying a 0).
 Unlike in the hard disk, the tracks represent a single spiral that moves outward from the center The disk rotates at high speed which, for CDs, is specified as a multiple of the speed of music CDs (150 KB/second). A music CD is rated as IX while data CDs are commonly rated as 52X

A CD-ROM has a capacity of 700 MB. A DVD-ROM which has its pits and lands much more closely spaced has a capacity of 4.7 GB. Addition of a second layer increases the capacity to 8.5 GB (a dual-layer DVD). Further, with the introduction of the “R” (recordable) and “RW” (rewritable) types, it is now possible to record data onto the media:

CD-R, DVD-R Data can be recorded only once.

CD-RW, DVD-RW Data can be recorded multiple times.

Initially, we had separate drives for reading and writing, but today a DVD writer can read and write both media. Even though CDs and DVDs are reliable media, they are likely to make way for flash memory which already supports much higher capacities without using any moving parts. The future of the Blu-ray disk for data backup is uncertain, even though it is extensively used by the music and film industries.

Flash Memory

The memory stick or pen drive is the most common type of flash memory used on the computer. It is a small, removable piece of circuit wrapped in a plastic casing. It connects to the USB port of the computer where it is detected as yet another drive by the operating system (say, 6: in Windows). Even though it is a read-write device, the number of writes is limited (around 10,000), which is adequate for most users. With 16 GB devices available at affordable prices, the USB pen drive has now become a serious competitor to the CD and DVD as an offline storage device.

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flash memory

Floppy Diskette

The floppy diskette was once the only form of portable storage that could be carried in the pocket. This storage medium is represented by a rectangular plastic case containing a thin magnetic disk A read/write head actually makes contact with this disk while it is rotating. The floppy was available in two sizes (5.25″ and 3.5″), offering capacities of 1.2 MB and 1.44 MB (yes, MB not GB), respectively.

The small capacity of a floppy diskette was enough for most uses in those days, but today a diskette can’t even hold a single 3-minute song encoded in MP3.


Devices like printers and scanners connect to a computer through docking points called ports. Current computers and laptops offer fewer types of ports today than they used to previously. All ports are connected to the motherboard but are visible from the outside. They are shaped differently, so it is impossible to use a wrong connector for a port. These are the ports that you’ll encounter:

  •  Universal Serial Bus (USB) The USB port has virtually replaced the serial and parallel ports in the motherboard. Most computers offer at least four USB ports to support scanners, printers and mice. A USB port has four lines, two cach for data and power. The current version, USB 3.0, can transfer a 1 GB file in 20 seconds. Also, a smaller variant, the micro-USB port, is used on portable hard disks and smartphones.
  • Serial port Once used by the keyboard, terminals, mice and modems, serial ports are offered in 9- and 25-pin configurations. Data pass through a serial port one bit at a time.


  • Parallel port If a printer is not using the USB port, then it is probably using the parallel port.The common implementation (introduced by IBM on the PC) uses 25 pins where data are transferred in parallel, i.e., synchronously.


  • Video Graphics Array (VGA) port This 15-pin port allows transfer of analog (continuous) video data to the monitor. This port is being replaced with the digital video interface (DVT) which uses digital data that is used by flat LCD panels (which also use analog signals). DVI offers better video resolution than VGA.


  • RJ45 port This port is used by the Ethernet network (1.14). Both computer and the network device (like hub or router) have the female form of the port. Even if the computer connects wirelessly to a network (using built-in Wi-Fi or a USB adapter), the wired RJ45 remains a useful option.


  • PS/2 port This port has replaced the serial port for connecting the keyboard and mouse. It has 6 pins but occurs as a pair in two different colors. The port and connectors for the keyboard are purple, while the mouse uses the green port. The laptop doesn’t support these ports but the desktop does. USB has invaded this area also.


  • High Definition Multimedia Interface (HDMI)This is now the industry standard for transferring audio and video data between computers and HDTVs, projectors and home theaters. Because a single cable handles both audio and video, it is driving out the RCA 3-plug system which all desktop PCs still support.
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ports and connectors


The input device is usually a keyboard where programs and data are entered in the computer. Examples of other input devices include a mouse, a pen or stylus, a touch screen or an audio input unit.

The Keyboard

Every computer supports a keyboard-either a physical one or a touchscreen. The central portion of the keyboard has the QWERTY layout and numerals are placed above the three layers of letters. There are also a large number of symbols that were never seen in the typewriter (like ^, – and “). All symbols seen on the keyboard have significance in C.

Each letter, numeral or symbol is known as a character, which represents the smallest piece of information that you can deal with. All of these characters have unique values assigned to them, called the ASCII value (ASCII-American Standard Code for Information Interchange). For instance, the letter A has the ASCII value 65, while the bang or exclamation mark (1) has the value 21. When you press A, the binary value of 65 is transferred to RAM. You must also know the function of the following special keys:

  • The [Enter] key terminates a line. The text that you key in remains hidden from the system until this key is pressed.


  • The backspace key erases input from right to left, while [Delete] erases text from left to right.


  • The (Ctrl] (called control) key, found in duplicate on the lowest row, is always used in combination with other keys. The [Ctrl-cl sequence (two keys) copies selected input, while[Ctrl-v] pastes it elsewhere.


  • The 12 function keys labeled [FI], [F2], etc. are located in the top-most row. Many application software (like Microsoft Word) make use of these keys.


The keyboard is an external device of the desktop which connects to it using the PS/2 port. Laptops and touchscreen-based computers don’t need this port because their keyboard is a builtin. If you need an external keyboard for your laptop, use a USB keyboard.

Pointing Devices

Graphical user interfaces (GUI) like Windows need a pointing device to control the movement of the cursor on the screen. This is commonly implemented as the mouse in the desktop and touchpad in the laptop. The earliest form of the mouse, one that is connected to a computer port, has a rotating ball at the bottom and two buttons at the top. Moving the mouse by hand partially rotates the ball and thereby controls the motion of the cursor on the screen. The buttons ate generally used in the following ways:

  • Clicking on any object with the left button selects that object. Double-clicking of the same button activates a program. A text section can be selected by dragging the mouse with the left button pressed (no clicking here). A left click on a text segment is also required to commence text entry .


  • Clicking on the right button activates a context-sensitive menu that can be used to change the attributes of the selected object. This menu often supports options for copying and pasting text and change the properties of the selected object.

The scroll wheel scrolls a document in two ways. You can keep the mouse stationery and move the wheel. Alternatively, click the wheel as you click a button and move the mouse a little The document then scrolls continuously without any further motion of the mouse.

Because of its usefulness, the mouse has undergone rapid changes in design. The original mechanical mouse is now being gradually phased out in favor of the following options:

  • The optical mouse uses an infrared laser or LED to track movement. Unlike the mechanical mouse, the optical mouse works practically on any surface.
  • The wireless mouse uses radio frequency technology to communicate with the computer The transmitter in the mouse communicates with a USB-connected receiver.


A scanner is a device that creates a digital image of a document by optically scanning it. Scanners come in various forms that are used in diverse industries. In this section, we examine the flatbed scanner that we all use for scanning documents not exceeding A4 size. This device connects to the USB port and is operated using a special software that is shipped with the product.

The document to be scanned is placed on a glass plate that is covered by a lid before scanning A shining light is beamed at the document and the reflected light is captured by a photosensitive array of charged coupled devices (CCD). The differing levels of brightness are converted into electronic signals. The signals are then processed to create the digital image which is saved as a file in the hard disk. The scanner can also act in photocopier mode and directly print the copied document without saving it.

Modern scanners have the Optical Character Recognition (OCR) facility by which a document (or its image) can be scanned in a special mode to extract the text as a stream of characters. This needs special software that can interpret the dot pattern in a picture as a character, say, A. The advantage in using OCR is that an image file can be converted to a text file which can be edited with text editing software. The file size also reduces substantially. If your existing scanner doesn’t have the  OCR function, then you need an OCR scanner. 


The output device is usually a monitor or a printer to show output. If the output is shown on the monitor, we say we have a soft copy. If it is printed on the printer, we say we have shard copy.

We generally use the term “output” to mean information that can be seen or heard. Printers and loudspeakers belong to this category. The scanner creates an image of a document but saves the “output” to disk. That, for us, is not output. In the following sections, we will discuss the commonly used visual output devices.

 The Monitor

The monitor is an integral part of a computer’s configuration. Programs use it to display text or graphical output, while users also need a monitor to view the input that they key in. The performance of a monitor is judged mainly by its image quality, resolution, energy consumption and its effect on the eyes. There are two competing technologies; the old CRT monitors have virtually made way for the modern LCD-based types. Monitors are connected to the desktop’s VGA or DVI port. The connection is internal for the laptop.

CRT Monitor

The CRT (cathode ray tube) monitor uses a rarefied tube containing three electron guns (for the three primary colors) and a screen coated with phosphorescent material. The guns emit electrons to create images on the screen by selectively lighting up the phosphors. CRT monitors typically have a resolution of 640 x 480 pixels which translates to an aspect ratio of 4:3 (Aspect ratio= width/height). They are large and heavy, energy-inefficient and generate a lot of heat.

LCD Monitors

Most computers today are shipped with LCD monitors (or their variants, LED and TFT). An LCD screen comprises thousands of liquid crystals, which, by themselves, don’t  generate light but may allow or block the passage of light through them. An image is formed by selectively applying a voltage to these crystals and using a separate light source for light to pa through them. The backlight is provided either by fluorescent light (the standard LCD) or by LED, (hence the term LED monitors and LED TV).

LCD monitors have a wider aspect ratio (16:9) compared to the 4:3 offered by CRT monitors From a generic standpoint, they have greatly improved upon the drawbacks of the CRT monitor They consume less power, generate less heat, have increased life span and take up a lot less space It is even possible to mount an LCD monitor on the wall. However, the viewing angle is less than 180 degrees, but this attribute is undergoing constant improvement.

Note: LEDs are simply backlight providers and have no role in the formation of the image The newer TFT (thin film transistor) is simply a variant of the basic LCD technology.

 Impact Printers

Printers produce hard copies of output (in contrast to disk files which are soft copies) which include text and graphics. There are currently two basic technologies used for printing-impact and non-impact. The older impact technology uses a printhead to strike (impact) a ribbon placed between the printhead and paper. Impact printers are noisy and are being phased out, but one variant (the dot-matrix printer) is still in active use.

Dot-matrix Printer

The printhead of the dot-matrix printer has either 9 or 24 pins which are fired in multiple combinations to generate letters, numerals and symbols. The ribbon is impregnated with ink, and when the pins fire against the ribbon, an impression is created on the paper behind it. The paper itself is moved forward by a drum after one line of printing has been completed. Because each pin can be controlled individually, it is possible to print graphics with this technologs However, multiple fonts can’t be used.

The speed of a dot-matrix printer can be as high as 300 cps (characters per second). A printer with 24 pins offers the best print quality (low as it is) at 144 dpi (dots per inch). However, the gaps between the pins are clearly visible, so this technology doesn’t produce quality output. This printer type refuses to go away simply because it can produce carbon copies on continuous stationery Invoices and bills are often printed in multiple copies, the reason why shops generally use dot-matrix printers

Daisy-wheel Printer

The now obsolete daisy-wheel printer uses a different technology to strike the ribbon. It employs a wheel with separate characters distributed along its outer edge The characters are thus pre-formed and not generated. For printing a character, the wheel is rotated so that the desired character directly faces the ribbon. You can’t print graphics with this printer but you can change the wheel to obtain a different set of fonts.

Line Printer

For heavy printing, the line printer is still the best bet. This technology The chain rotates chain containing the characters. continuously in front of the paper. Hammers strike the paper in the normal manner, but the print speeds attained with this technology a as high as 1200 lpm (lines per minute). Line printers are used to print low-quality voluminous uses a print reports and can print unattended for long stretches of time. However, a line printer is extremely noisy, the reason why it is generally kept in a separate room.

 Non-Impact Printers

Non-impact printers address the drawbacks of impact printers. They are quiet, fast and produce documents of very high resolution. The two types that are most commonly used are laser printers and inkjet printers. The third type not discussed here is the thermal printer which uses heat to print text and images on heat-sensitive paper.

Laser Printer

A laser printer works somewhat like a photocopier. A laser beam creates an image of the page to be printed on a light-sensitive drum. The charged areas attract black magnetic powder, called the toner. The image, which is created in the form of dots, is then transferred from the drum to the paper by actual contact. A separate roller heats up the paper to melt the toner which then gets fused onto the paper. Color laser printers use colored toner.

Laser printers have built-in RAM for storing documents. This RAM acts as a buffer which can free up the computer after a job has been submitted for printing. A laser printer also has a ROM to store fonts. This technology is suitable for printing text and graphics in high quality. The resolution varies from 300 dpi (dots per inch) to 1200 dpi. Speeds of 20 ppm (pages per minute) are typical,while the fastest printers operate at 200 ppm. Originally used for desktop publishing, falling prices have made the laser printer available everywhere.

Ink-jet Printer

The ink-jet printer is an affordable non-impact printer whose print quality considerably outperforms a dot-matrix printer but under performs a laser printer. In this system, a printhead sprays tiny drops of ink at high pressure as it moves along the paper. The ink, stored in a replaceable cartridge, passes through a matrix comprising a number of tiny nozzles. Like with the pins of a dot-matrix printer, characters are formed by choosing the nozzles that have to be activated. Color ink-jet printers either have separate cartridges for each color or use a multi-chambered cartridge.

Inkjet printers typically have a resolution of 300 dpi, but their speed is low-around 1 to 6 pages per minute. They are thus not suitable for high-volume printing where laser printers offer a better option. Better print quality can be obtained by using high-quality acid-free paper that can retain the quality even after years of storage.


Unlike printers which print text and graphics, a plotter makes line drawings. It uses one or more automated pens to complete each line before taking up the next one. The commands are taken from special files called vector graphic files. Depending on the type of plotter, either the plotter pen moves or the paper moves or both. Plotters can handle large paper sizes, and are thus suitable for creating drawings of buildings and machines. They are also slow and expensive, the reason why wide-format printers are increasingly replacing them.

Note: Programs also take input from and write output to the hard disk. For convenience and to avoid conflict in definition, we don’t consider the disk as an input/output device. We use the term

Auxiliary storage

Auxiliary storage, or secondary storage, is used for both input and output. It is the place where the programs and data are stored permanently. When you turn off the computer, your programs and data remain in the secondary storage ready for the next time you need them.

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