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UNIT 1 ▪ CHAPTER 1
01
Basic Computer Organisation
Hardware · Software · CPU · Memory
A Computer is an electronic device that accepts input (data & instructions), processes it according to a stored set of instructions (called a program), stores the data and results, and produces meaningful output. The word comes from the Latin computāre, meaning "to calculate" — but today's computers do far more than arithmetic. They play music, drive cars, detect diseases, and even write poetry. In this chapter we study the machine itself: the parts it is built from, how those parts cooperate, the different kinds of software that make it useful, and the operating system that ties everything together.
Key characteristics of a computer:
- Speed — modern processors execute billions of instructions per second.
- Accuracy — output is exactly what the program asks for (errors are usually human or input errors).
- Storage — can hold vast amounts of data (photos, videos, entire libraries).
- Diligence — performs the same task repeatedly without getting tired or bored.
- Versatility — the same device can compose music, calculate GST, and stream a movie.
- Automation — once programmed, it works without human intervention.
Learning Outcome 1: Describe the functional units of a computer system and the devices connected to it.
1.1 Basic Computer Organisation
A computer system is organised into four functional units that work together. Input travels from the outside world into the machine, gets processed by the CPU with the help of memory, and output goes back out. This design is often called the von Neumann architecture (after mathematician John von Neumann, 1945).
Block diagram (conceptual):
The four units — Input, CPU, Memory, Output — plus a system bus that carries data between them, together make up the computer. The arrows show the direction of normal data flow; the CPU↔Memory link is two-way because the CPU both reads instructions from memory and writes results back to it.
1.2 Hardware vs. Software
Everything physical you can touch in a computer is hardware. Everything you can only run — the instructions that tell the hardware what to do — is software. A computer needs both to be useful. A laptop without Windows is a brick; Windows without a laptop is a DVD.
| Aspect | Hardware | Software |
|---|---|---|
| Nature | Physical / tangible | Logical / intangible (programs) |
| Touch? | Yes — can be seen & held | No — stored as data / code |
| Wear & tear | Suffers physical damage | Does not wear out, but can become outdated or corrupted |
| Examples | CPU, RAM, keyboard, monitor | Windows, MS Word, Chrome, Python |
| Replacement | Must be physically installed | Downloaded / re-installed digitally |
1.3 Input Devices
An input device is any hardware that lets the user feed data or instructions into the computer. It converts human-readable information into the binary signals the CPU understands.
1.3.1 Common Input Devices
| Device | What it does | Everyday Example |
|---|---|---|
| Keyboard | Enters text and commands by pressing keys (QWERTY layout, 104/108 keys). | Typing an essay in MS Word. |
| Mouse | A pointing device — moves a cursor, clicks and drags. | Selecting a file on the desktop. |
| Touch screen | A screen that senses finger or stylus touch — acts as input and output. | Android phone, ATM, self-order kiosk. |
| Scanner | Converts a printed document or photo into a digital image. | Scanning an Aadhaar card for upload. |
| Microphone | Converts sound waves into an audio signal. | Voice note on WhatsApp. |
| Webcam / Camera | Captures still photos and live video. | Attending a Google Meet class. |
| Joystick / Gamepad | Lets the user control movement in games or simulators. | Flight simulator, console gaming. |
| Light pen | A pen-shaped sensor used to draw or select directly on the screen. | Designing on a graphics tablet. |
| Barcode reader | Reads the black-and-white bars printed on products. | Billing at a grocery store POS. |
| OMR (Optical Mark Reader) | Detects pencil marks on a printed sheet. | Checking JEE / NEET OMR answer sheets. |
| Biometric sensor | Reads a unique body trait — fingerprint, iris, face. | Aadhaar authentication, office attendance. |
1.4 Output Devices
An output device presents processed information back to the user, usually by converting the computer's binary signals into a form humans can see, hear or feel.
1.4.1 Visual Output Devices
- Monitor / Display — the most common visual output. Types:
- CRT (Cathode Ray Tube) — bulky, older TVs; rarely used now.
- LCD (Liquid Crystal Display) — thin, uses fluorescent backlight.
- LED (Light Emitting Diode) — an LCD with LED backlight; brighter, more energy-efficient.
- OLED — each pixel emits its own light; richer colours, found in premium phones and TVs.
- Projector — throws the computer's output onto a large screen or wall; used in classrooms and theatres.
- Printer — produces a hard copy on paper. Two broad families:
- Impact printer — a print head strikes an inked ribbon onto the paper (e.g., dot-matrix). Noisy but cheap per page; still used for railway reservation chart and GST invoices with carbon copies.
- Non-impact printer — no physical contact between head and paper. E.g., inkjet (sprays liquid ink — good for photos) and laser (uses a laser + toner — fast, sharp text; standard in schools and offices).
- Plotter — a special printer for very large, accurate diagrams (engineering drawings, architectural blueprints).
1.4.2 Audio Output Devices
- Speakers — convert the computer's electrical audio signal into sound.
- Headphones / earphones — personal audio for private listening.
Input or output? A few devices do both:
- Touch screen — shows output and accepts touch input.
- Modem — sends (output) and receives (input) network data.
- Headset with mic — speaker = output, mic = input.
1.5 The CPU — Central Processing Unit
The CPU is the "brain" of the computer — the chip that actually carries out the instructions of a program. In a modern laptop it is a small square chip, typically made by Intel (Core i3 / i5 / i7) or AMD (Ryzen). Mobile phones use chips such as Qualcomm Snapdragon or Apple's A-series. A CPU has three main parts:
ALU — Arithmetic & Logic Unit
Performs all calculations (add, subtract, multiply, divide) and comparisons (greater than, equal, less than). This is where real "thinking" happens.
CU — Control Unit
Fetches each instruction from memory, decodes what it means, and tells the ALU, memory and I/O what to do. Think of it as the traffic policeman of the chip.
Registers
Tiny, ultra-fast storage inside the CPU. Hold the instruction being executed and the data being worked on right now. Access time: < 1 ns.
The Fetch–Decode–Execute cycle. Every instruction a computer runs follows the same three-step pattern, millions or billions of times per second:
- Fetch — the CU pulls the next instruction from main memory into a register.
- Decode — the CU figures out what the instruction is (add? move? jump?).
- Execute — the ALU (or another unit) actually performs the operation and writes the result back.
Fetch – Decode – Execute cycle (visual):
Learning Outcome 2: Classify different types of memory and state their units.
1.6 Memory of a Computer
Computers need places to keep data and instructions. These storage areas are collectively called memory. But not all memory is the same — it differs in speed, size and cost. Memory is organised as a hierarchy, with a small amount of very fast, costly memory close to the CPU, and a large amount of slower, cheaper memory further away.
Memory Hierarchy (pyramid — fastest at top):
1.6.1 Primary Memory (Main Memory)
Primary memory is directly accessible by the CPU. It holds the program currently running and the data it is working on. When you open Chrome, Windows copies Chrome from your hard disk into primary memory so the CPU can execute it quickly.
(a) RAM — Random Access Memory
- Volatile — data is lost the moment power is switched off.
- Read and write both are possible.
- Used as the computer's working memory.
- Two sub-types:
- SRAM (Static RAM) — faster, used inside cache; costlier.
- DRAM (Dynamic RAM) — cheaper, used as main RAM sticks (e.g., 8 GB DDR4).
(b) ROM — Read Only Memory
- Non-volatile — keeps its contents even when power is off.
- Normally only read, not written to during everyday use.
- Stores start-up instructions — the BIOS firmware that runs the moment you press the power button.
- Sub-types: PROM (Programmable — written once by the user), EPROM (Erasable using UV light, re-writable), EEPROM (Electrically erasable — today's BIOS chips).
| Feature | RAM | ROM |
|---|---|---|
| Volatility | Volatile (loses data on power-off) | Non-volatile (retains data) |
| Read/Write | Read & Write | Mostly Read only |
| Purpose | Working memory for running programs | Start-up firmware (BIOS) |
| Typical size | 4 GB – 32 GB | A few MB |
| Modifiable by user? | Yes — data changes constantly | Usually no (manufacturer-written) |
1.6.2 Cache Memory
Cache is a small but very fast SRAM memory placed between the CPU and the main RAM. It stores copies of the data the CPU is likely to use next — so the CPU doesn't have to wait for slow RAM every time. Most modern processors have three levels:
- L1 cache — tiny (e.g., 64 KB), built right inside each core, the fastest.
- L2 cache — a few hundred KB, also per-core.
- L3 cache — larger (several MB), shared by all cores.
Why cache? Accessing RAM takes about 100 CPU cycles; accessing L1 cache takes 2–4. A well-designed cache reduces this delay by keeping recently used and nearby data close at hand — an idea called the locality of reference.
1.6.3 Secondary Memory (Auxiliary Storage)
Secondary memory keeps data permanently, even when the machine is off. It is slower than RAM, but far larger and cheaper. This is where your photos, documents, installed apps and OS itself actually live.
| Device | Technology | Typical Capacity | Common Use |
|---|---|---|---|
| HDD (Hard Disk Drive) | Spinning magnetic platters + moving head | 500 GB – 10 TB | Bulk storage in desktops & servers |
| SSD (Solid State Drive) | Flash memory chips (no moving parts) | 128 GB – 4 TB | Fast boot drives in modern laptops |
| USB pen drive | Flash memory | 8 GB – 1 TB | Carrying files between computers |
| Memory card (SD / microSD) | Flash memory | 16 GB – 1 TB | Phones, cameras, Raspberry Pi |
| CD / DVD / Blu-ray | Optical — laser reads pits on a disc | 700 MB / 4.7 GB / 50 GB | Movies, software distribution (legacy) |
| Magnetic tape | Long tape coated with magnetic particles | Hundreds of TB per cartridge | Enterprise back-ups, archives |
| Cloud storage | Remote servers accessed over the Internet | Effectively unlimited | Google Drive, DigiLocker, DropBox |
1.6.4 Units of Memory
Computers store everything — text, images, sounds, videos — as binary digits (0 and 1). A single binary digit is a bit. Larger amounts of memory are expressed as multiples of a byte.
| Unit | Symbol | Size | Roughly… |
|---|---|---|---|
| Bit | b | 0 or 1 | Smallest unit — a single yes/no |
| Nibble | — | 4 bits | Half a byte (one hex digit) |
| Byte | B | 8 bits | 1 character (e.g., the letter 'A') |
| Kilobyte | KB | 1024 bytes | ½ page of typed text |
| Megabyte | MB | 1024 KB | A short MP3 song |
| Gigabyte | GB | 1024 MB | A full-length HD movie |
| Terabyte | TB | 1024 GB | Capacity of a laptop hard disk |
| Petabyte | PB | 1024 TB | Data processed by Google in ~a day |
| Exabyte | EB | 1024 PB | Total global Internet traffic every few hours |
Memory conversion shortcut:
1 KB = 210 B = 1024 B
1 MB = 220 B = 1024 KB
1 GB = 230 B = 1024 MB
1 TB = 240 B = 1024 GB
Each step up is × 1024 (≈ 1000).
1 KB = 210 B = 1024 B
1 MB = 220 B = 1024 KB
1 GB = 230 B = 1024 MB
1 TB = 240 B = 1024 GB
Each step up is × 1024 (≈ 1000).
Quick calculations:
- A pen drive is labelled 16 GB. How many bytes is that? → 16 × 1024 × 1024 × 1024 ≈ 1.72 × 1010 bytes (~17 billion).
- An MP3 song is 4 MB. How many songs fit in 1 GB? → 1024 ÷ 4 = 256 songs.
- A photo is 2 MB. How many photos fit in an 8 GB card? → 8192 ÷ 2 = 4096 photos.
📌 Quick Revision — Chapter 1 at a Glance
- Computer = electronic device that processes data following stored instructions.
- Four functional units: Input → CPU (ALU + CU + Registers) → Output, with Memory attached (von Neumann architecture).
- Hardware = physical parts; Software = programs. Both required.
- Input devices: keyboard, mouse, scanner, mic, webcam, touch screen, barcode, OMR, biometric.
- Output devices: monitor (CRT / LCD / LED / OLED), printer (impact vs non-impact — inkjet, laser), projector, speaker, plotter.
- CPU = ALU + CU + Registers. Follows Fetch-Decode-Execute cycle. Speed in GHz.
- Memory hierarchy: Registers → Cache (L1/L2/L3) → Primary (RAM, ROM) → Secondary (HDD, SSD, USB, cloud).
- RAM = volatile, read/write. ROM = non-volatile, mainly read, stores BIOS.
- Units: 1 byte = 8 bits; × 1024 each step → KB, MB, GB, TB, PB.