Computer System: Hardware and Software

A computer is an electronic device that accepts data, processes it according to stored instructions, stores the result and produces output in a useful form. Every computer — a laptop in a school lab, a mobile phone, or the server that runs the IRCTC ticketing website — is built on the same arrangement of parts. Textbooks call this the block diagram of a computer, and you should be able to draw it and label it from memory.

Two words are tested together here. Data means raw, unprocessed facts and figures — the marks 78, 91 and 64. Information is processed data that carries meaning — the average mark, 77.67. The whole purpose of a computer is to convert data into information.

The block diagram has four units.

1. Input unit. It accepts data and instructions from the user, converts them into the binary form the machine understands (only 0 and 1) and passes them to memory. Keyboards, mice and scanners are input devices.

2. Central Processing Unit (CPU). Often called the brain of the computer, it carries out all processing and controls every other unit. It has two main parts. Many textbooks show primary memory as a third part inside the CPU block, so follow the version your class notes use.

• Arithmetic Logic Unit (ALU) — performs all arithmetic operations (addition, subtraction, multiplication, division) and all logical operations (comparisons such as greater than, less than and equal to, and AND, OR, NOT decisions). Every actual calculation happens here.
• Control Unit (CU) — does not process data itself. It fetches instructions from memory, decodes them, and sends timing and control signals telling the other units what to do and when. It is the manager, not the worker.

The CPU also contains a few very fast storage locations called registers, which hold the data an instruction is working on at that instant.

3. Memory unit. Primary memory holds the program being run and the data being worked on, and the CPU can reach it directly. Secondary storage keeps data permanently, but the CPU cannot use it directly — data must first be copied into primary memory.

4. Output unit. It takes the result from memory, converts it from binary into a human-readable form, and presents it on a monitor, printer or speaker.

When you draw the diagram, show solid arrows for the flow of data (input to memory, memory to ALU, ALU back to memory, memory to output) and dashed arrows for control signals running from the control unit to every other unit. A common mistake is to draw the control unit with no arrows reaching the input and output units.

Trace one calculation to fix the sequence in your mind. You type 2500 + 1750 on the keyboard, which is input. The numbers are stored in primary memory. The CU fetches the addition instruction, decodes it and signals the ALU. The ALU adds and sends 4250 back to memory. The CU then signals the output unit, and 4250 appears on the screen. Notice that the ALU never talks to the keyboard directly — everything moves through memory.

Finally, the idea that makes this a computer rather than a calculator is the stored program concept: data and program instructions are held in the same memory, so the machine takes on a completely new job when a new program is loaded, instead of having to be rewired.

A peripheral is any device attached to a computer that is not part of the CPU or main memory. Peripherals are grouped as input devices, output devices, or devices that do both.

Input devices accept data and instructions from the user and convert them into binary form.

• Keyboard — the standard text input device, usually in the QWERTY layout. Its keys are grouped as alphanumeric keys, a numeric keypad, function keys (F1 to F12), navigation keys and special keys such as Ctrl, Alt and Esc.
• Mouse — a pointing device that moves the on-screen cursor and selects items by clicking, double-clicking and dragging. An optical mouse uses a light source and sensor instead of a rolling ball.
• Scanner — converts a printed document or photograph into a digital image, for example when a school digitises old mark sheets.
• Optical Mark Reader (OMR) — detects pencil marks made in fixed positions on a printed sheet. This is how large entrance-test answer sheets are evaluated quickly.
• Optical Character Recognition (OCR) — works with a scanner to read printed or typed characters from a page and turn the image into editable text.
• Magnetic Ink Character Reader (MICR) — reads the special magnetic-ink digits printed along the bottom of a bank cheque, letting banks clear cheques quickly and without typing errors.
• Barcode reader — reads the pattern of thick and thin bars printed on product packaging at a billing counter.
• Biometric devices — fingerprint and iris scanners read a physical characteristic of a person. Aadhaar-based authentication at a bank or a fair price shop works this way.
• Microphone, webcam, joystick, light pen, digital camera and the graphics tablet used for freehand drawing.

Output devices take processed results from memory and present them in human-readable form. Output is of two kinds. Soft copy is temporary output that you cannot touch — what appears on a monitor or comes out of a speaker. Hard copy is permanent output on paper, produced by a printer or plotter.

• Monitor, also called the VDU (Visual Display Unit) — LCD and LED monitors have largely replaced the older bulky CRT type. Sharpness depends on resolution, the number of picture elements or pixels that make up the screen.
• Printers fall into two families. An impact printer strikes the paper through an inked ribbon, so it is noisy but can produce carbon copies; the dot-matrix printer is the standard example and is still used where multi-part bills are printed. A non-impact printer does not strike the paper. Inkjet printers spray very fine droplets of ink and give good colour at a low purchase price. Laser printers use a laser beam and toner powder, and are fast, quiet and economical for large volumes of text.
• Plotter — draws large, accurate line drawings such as building plans and maps.
• Speakers and headphones give audio output, and a projector throws the display onto a screen for a whole classroom.

Some devices are both input and output. A touchscreen displays output and accepts touch input. A modem both sends and receives data, and a headset with a built-in microphone does both.

Two points are commonly muddled. A storage device such as a pen drive is classified as secondary storage, not as an input or output device, even though data travels both ways. And hardware means the physical parts you can touch, while software is the set of programs that tells the hardware what to do.

Primary memory, also called main memory or internal memory, is the memory the CPU can read from and write to directly. A program must be loaded into primary memory before it can run. Primary memory is faster than secondary storage but much smaller in capacity and, in the case of RAM, far more expensive per unit of storage.

RAM (Random Access Memory) is the working memory of the computer. When you open a spreadsheet of fee records, a copy of that file and a copy of the spreadsheet program are loaded from the hard disk into RAM, and the CPU works on that copy. RAM is called random access because any location can be reached directly in the same amount of time, without reading through everything before it.

The defining property of RAM is that it is volatile: its contents are erased the moment the power is switched off. This is exactly why unsaved work is lost in a power cut, and why every computer also needs secondary storage. RAM is a read/write memory — data can be both stored and changed.

RAM is of two types.

• DRAM (Dynamic RAM) — stores each bit in a tiny capacitor that leaks charge, so its contents must be refreshed constantly. It is slower but cheaper and denser, so it is used as the computer's main RAM.
• SRAM (Static RAM) — stores each bit in a flip-flop circuit and needs no refreshing. It is faster, but is costlier and takes more space per bit, so it is used only in small amounts, mainly as cache memory.

ROM (Read Only Memory) holds instructions that the computer needs but that must never be changed by ordinary use. Its contents are written in by the manufacturer and can normally only be read, not written to. ROM is non-volatile: the contents survive when the power goes off. The classic example is the start-up program, often called the BIOS (Basic Input Output System), which runs the moment you press the power button, checks the hardware and then loads the operating system from disk. ROM is also used in embedded devices such as washing machines, microwave ovens and calculators, where the program never changes.

ROM has three named types in the syllabus.

• PROM (Programmable ROM) — supplied blank; the user can write data into it once using a special device. After that it cannot be changed.
• EPROM (Erasable Programmable ROM) — can be erased by exposing the chip to ultraviolet light through a small quartz window, and then reprogrammed.
• EEPROM (Electrically Erasable Programmable ROM) — can be erased and rewritten electrically, without removing the chip. Flash memory is built on this idea, which is why the firmware in most modern machines can be updated by software.

Two more small but fast memories belong to this discussion. Cache memory sits between the CPU and RAM and holds the instructions and data used most often, so the CPU does not have to wait for slower RAM. Registers are inside the CPU itself and are the fastest storage of all, holding only the few values being operated on right now.

Learn the speed ladder in order, because comparison questions rely on it: registers are fastest, then cache, then RAM, then secondary storage, and capacity and cost per unit run the other way — secondary storage is the largest and cheapest per gigabyte.

Secondary storage, also called auxiliary or external storage, keeps programs and data permanently. It is non-volatile, so nothing is lost when the power is switched off, and it offers far more capacity at a far lower cost per gigabyte than RAM. Its one limitation is that the CPU cannot use it directly: the data must first be copied into primary memory.

Hard Disk Drive (HDD). The traditional secondary storage of a desktop or laptop. It contains one or more rigid circular metal platters coated with a magnetic material, spinning at high speed, with a read/write head moving across each surface on an arm. Each surface is divided into concentric circles called tracks, and each track is divided into sectors. Data is stored as patterns of magnetisation. An HDD offers large capacity at low cost, but because it has moving parts it is slower and can be damaged if the machine is dropped or knocked while running. An external hard disk is the same technology in a portable case connected through a USB port.

Solid State Drive (SSD). An SSD stores data in flash memory chips and has no moving parts at all. Compared with an HDD of the same capacity it is much faster to start up and to open files, silent, lighter, and far more resistant to shock — but it costs more per gigabyte. The key contrast to remember is that an HDD is magnetic with moving parts, while an SSD is electronic with none.

Optical storage. Optical discs store data as microscopic pits and lands on a reflective spiral track, read by a laser beam. In increasing capacity they are the CD (Compact Disc), the DVD (Digital Versatile Disc) and the Blu-ray Disc, whose blue-violet laser has a shorter wavelength and so can read much finer pits. Each comes in three forms:

• ROM — for example CD-ROM: written by the manufacturer, can only be read.
• R (recordable) — for example DVD-R: can be written by the user once, then only read.
• RW (rewritable) — for example CD-RW: can be erased and rewritten many times.

Optical discs are cheap and easy to distribute but slow and easily scratched, and are being replaced by flash and network storage.

Flash memory devices. These use EEPROM-based flash chips, so like an SSD they have no moving parts. The pen drive, also called a USB flash drive or thumb drive, plugs into a USB port and is the usual way a student carries a project between home and school. Memory cards such as SD and microSD cards are used in cameras and mobile phones. Flash storage is compact, quick and sturdy, but the chips have a limited number of write cycles and small drives are easy to lose.

Two access methods complete the topic. Sequential access means records must be read one after another from the beginning, as on a magnetic tape used for bulk backups. Direct or random access means any record can be reached straight away, as on a hard disk, SSD or optical disc. Direct access is far faster for everyday work, which is why tape survives only for archival backup.

You may also meet cloud storage, where files are kept on servers reached over the internet. It gives access from any device and protects against loss of the physical machine, but it needs a working internet connection and raises questions about who else can see your data.

A computer stores everything — text, numbers, pictures, sound — as patterns of two states, written as 0 and 1. A single 0 or 1 is a bit, short for binary digit, and it is the smallest unit of storage there is.

Bits are grouped for convenience. Four bits make a nibble. Eight bits make a byte, and the byte is the basic unit of memory measurement, because one byte is enough to store one character — one letter, one digit or one punctuation mark — in the older character codes. So the word INDIA occupies 5 bytes.

Larger units go up in steps of 1024, not 1000. The reason is that computers count in powers of 2, and 1024 is 2¹⁰, the power of 2 closest to a thousand. Learn this ladder in order:

• 1 nibble = 4 bits
• 1 byte (B) = 8 bits
• 1 kilobyte (KB) = 1024 bytes = 2¹⁰ bytes
• 1 megabyte (MB) = 1024 KB = 2²⁰ bytes
• 1 gigabyte (GB) = 1024 MB = 2³⁰ bytes
• 1 terabyte (TB) = 1024 GB = 2⁴⁰ bytes

Above the terabyte the same pattern continues with the petabyte (1024 TB), exabyte and beyond.

Conversions are pure arithmetic, and the only rule you need is this: going down the ladder to a smaller unit, multiply by 1024; going up to a larger unit, divide by 1024. Work through these.

Example 1. Convert 6 GB into MB. You are moving to a smaller unit, so multiply: 6 x 1024 = 6144 MB.

Example 2. Convert 3 MB into bytes. Two steps down the ladder, so multiply twice: 3 x 1024 x 1024 = 3145728 bytes.

Example 3. Convert 8192 KB into MB. You are moving up, so divide: 8192 / 1024 = 8 MB.

Example 4. Convert 2 TB into GB. 2 x 1024 = 2048 GB.

Example 5. How many bits are there in 1 KB? First to bytes, then to bits: 1024 x 8 = 8192 bits.

Example 6 (the application type). A pen drive has 2 GB of free space. How many photographs of 4 MB each can it hold? Convert to the same unit first: 2 GB = 2 x 1024 = 2048 MB, and 2048 / 4 = 512 photographs. Always convert both quantities to the same unit before dividing — that single habit prevents most mistakes in this question type.

One real-world detail is worth knowing, and it explains a puzzle every student meets. Storage manufacturers measure capacity in powers of 10, taking 1 GB as 1,000,000,000 bytes, while the operating system reports capacity in powers of 2, taking 1 GB as 1,073,741,824 bytes. That is why a pen drive sold as 8 GB shows only about 7.45 GB of usable space. Nothing is missing; the two sides are simply counting with different-sized units. For your exam, always use the 1024-based values unless a question states otherwise.

Finally, do not confuse storage with speed. File sizes are measured in bytes, written with a capital B, while transfer speeds are measured in bits per second, written with a small b — so a 100 Mbps connection delivers at most about 12.5 MB per second, since you divide by 8.

Software is a set of instructions, called a program, that tells the hardware what to do. Hardware without software is an inert box; software without hardware has nothing to run on. Software is classified into three groups, and being able to place any named program in the right group is the skill tested here.

1. System software controls and manages the computer's own hardware and provides the platform on which everything else runs. It is generally supplied with the machine and works in the background rather than doing any job for the user directly.

The most important system software is the operating system (OS) — the master program that loads first, controls all hardware and runs every other program. Its main functions are: process management (deciding which program gets the CPU and when), memory management (allocating RAM to programs and reclaiming it), file management (organising files and folders on storage, and controlling who may open them), device management (communicating with the keyboard, printer and other peripherals through small programs called device drivers), and providing a user interface. Windows, Linux, macOS and Android are operating systems.

An interface can be of two kinds. A CLI (Command Line Interface) takes typed commands and is fast for an expert but has to be memorised. A GUI (Graphical User Interface) uses windows, icons, menus and a pointer, so it is far easier for a beginner, at the cost of more memory and processing power.

System software also includes language translators, which convert a program written by a human into machine language. An assembler converts assembly language into machine code. A compiler translates the whole high-level program at once and reports all the errors together, producing a separate executable file. An interpreter translates and runs one statement at a time and stops at the first error, which makes it slower to run but easier to learn with. Python is normally run through an interpreter.

2. Application software is written to do a particular job for the user. It is subdivided further:

• General purpose (packaged) software — sold ready-made for anyone to use: word processors, spreadsheets, presentation software, image editors, web browsers and media players.
• Customised (tailor-made) software — written for one organisation's specific need, for example a school's own fee-collection and result-processing system, or a bank's core banking application.

If a program does a job that a human wanted done — typing a letter, keeping accounts in rupees, editing a photograph — it is application software.

3. Utility software is the awkward middle category, and the one students most often misclassify. Utilities are small programs that help maintain, tune and protect the computer and its files. They do not run the machine like an OS, and they do not do the user's own work like an application. Standard examples are antivirus software, disk defragmenters that rearrange scattered file fragments on a hard disk, backup software, file compression tools that shrink files into a zip archive, disk cleanup tools that delete temporary files, and disk formatting tools. Many textbooks treat utilities as a sub-category of system software, so it is worth saying so in your answer.

Firmware completes the picture: software permanently stored in ROM inside a hardware device, such as the BIOS on a motherboard or the program inside a washing machine's controller. It sits between hardware and software, which is where its name comes from.

Every piece of software comes with a licence — a legal agreement stating what you are allowed to do with it. Software is classified by what that licence permits, and the deciding question is whether you get the source code: the human-readable instructions the programmer wrote, before they are translated into machine language.

Open source software is distributed with its source code, under a licence that allows anyone to study it, modify it, and redistribute the original or the modified version. The community can find and fix faults, and the software can be adapted to local needs, which is one reason many public institutions in India prefer it. Common examples are the Linux operating system, LibreOffice and OpenOffice office suites, the Mozilla Firefox browser, GIMP for image editing, Audacity for audio editing, the Python language and the MySQL database. Well-known open source licences include the GNU GPL (General Public License), the MIT licence and the Apache licence.

Proprietary software, also called closed source or commercial software, is owned by an individual or a company. The source code is kept secret, and the licence forbids you from copying, modifying or redistributing it. You do not buy the software itself; you buy the right to use it, usually for a fee and often on a limited number of machines. Examples are Microsoft Windows, macOS, Microsoft Office, Adobe Photoshop and accounting packages such as Tally. In return for the cost and the restrictions, the user generally gets official technical support, professional documentation and a company that is accountable for faults.

Now the distinction that students most often get wrong. Free does not mean free of cost. In the phrase free and open source software, often shortened to FOSS, the word free refers to freedom — the freedom to run, study, change and share the program — not to price. Open source software may legally be sold, and companies do sell supported versions of it. Equally, software that costs nothing is not automatically open source: a program can be given away with its source code locked shut.

That leads to three smaller categories:

• Freeware — available free of cost, for unlimited use, but the source code is not released and modification is not permitted. It is free, but it is proprietary.
• Shareware — distributed free for a trial period or with some features disabled, after which you are expected to pay to continue or to unlock the full version.
• Public domain software — the author has surrendered all copyright, so anyone may use it in any way at all, including inside commercial products.

Compare the two main models on five points. Cost: open source is usually free or low cost, proprietary is generally paid. Source code: available and modifiable, versus hidden. Support: community forums and documentation, versus a paid official channel. Updates: fixes from anyone in the community, versus the vendor alone. Customisation: free to adapt, versus not permitted.

Neither model is simply better. A school computer lab may run an open source operating system and office suite to avoid licence costs, while the same school's accounts office prefers a proprietary package because it comes with a support number to call. If a question asks which is better, give a balanced answer: it depends on budget, on the need for support and on the need to modify.