Unit 1: Introduction to Computers
📚 What is a Computer?
A computer is an electronic device that accepts input, processes it according to a set of instructions (program), and produces output. It can also store data for future use.
Input → Processing → Output → Storage
Characteristics of a Computer:
| Feature | Explanation |
|---|---|
| Speed | Performs billions of operations per second (measured in GHz) |
| Accuracy | Near-perfect accuracy — errors are from human input, not computer |
| Storage | Can store massive amounts of data (terabytes and beyond) |
| Diligence | Never gets tired or bored — can work 24/7 without errors |
| Versatility | Can perform many different tasks (gaming, calculations, design, etc.) |
| No Intelligence | Cannot think on its own — strictly follows instructions given by humans |
Generations of Computers:
| Gen | Period | Technology | Example | Key Features |
|---|---|---|---|---|
| 1st | 1940-56 | Vacuum Tubes | ENIAC, UNIVAC | Room-sized, very hot, unreliable, machine language only |
| 2nd | 1956-63 | Transistors | IBM 1401 | Smaller, faster, assembly language, magnetic storage |
| 3rd | 1964-71 | ICs (Integrated Circuits) | IBM 360 | Even smaller, high-level languages (FORTRAN, COBOL) |
| 4th | 1971-present | Microprocessors (VLSI) | Intel, Apple | Personal computers, GUI, internet, very cheap |
| 5th | Present+ | AI, Quantum Computing | IBM Watson | Artificial intelligence, natural language processing |
Types of Computers:
- By Size: Supercomputer (weather, nuclear research) > Mainframe (banking) > Minicomputer > Microcomputer (laptop, desktop) > Embedded (inside devices like TV, car)
- By Purpose: General purpose (multiple tasks) vs Special purpose (single task, e.g., ATM)
- By Data Type: Analog (continuous data) vs Digital (discrete 0s and 1s) vs Hybrid (combination)
📚 Hardware Components
Input Devices (data enters the computer):
| Device | Function |
|---|---|
| Keyboard | Type text, numbers, commands |
| Mouse | Point, click, select, drag |
| Scanner | Converts physical documents to digital images |
| Microphone | Audio input for voice recording/commands |
| Webcam | Captures video input |
| Barcode Reader | Reads barcodes on products (supermarkets) |
| OCR (Optical Character Recognition) | Reads printed text from images/scanned documents |
Output Devices (results come out):
| Device | Function |
|---|---|
| Monitor | Displays text, images, video (soft copy) |
| Printer | Produces paper output (hard copy). Types: Inkjet, Laser, Dot Matrix |
| Speaker | Audio output |
| Projector | Projects display on large screen/wall |
| Plotter | Prints large engineering/architectural drawings |
CPU (Central Processing Unit) — "The Brain":
The CPU has 3 main parts:
- ALU (Arithmetic Logic Unit): Performs all arithmetic (+, −, ×, ÷) and logical (AND, OR, NOT, comparisons) operations
- CU (Control Unit): Directs and coordinates all computer operations. Fetches instructions, decodes them, sends signals to other components. Like a traffic policeman.
- Registers: Tiny, extremely fast memory locations INSIDE the CPU. Used to hold data being
processed right now.
- Accumulator (AC): Stores intermediate calculation results
- Program Counter (PC): Holds address of NEXT instruction to execute
- Instruction Register (IR): Holds the CURRENT instruction being executed
- MAR (Memory Address Register): Holds the address of memory to read/write
- MDR (Memory Data Register): Holds data being read from or written to memory
Instruction Cycle (Machine Cycle):
Fetch → Decode → Execute → Store (repeats for every
instruction)
- Fetch: CU gets the next instruction from memory (using PC)
- Decode: CU interprets what the instruction means
- Execute: ALU performs the operation
- Store: Result is written back to memory or register
📚 Memory Hierarchy
Computer memory is organized in a pyramid — faster memory is smaller and more expensive.
| Level | Type | Speed | Size | Persistent? | Examples |
|---|---|---|---|---|---|
| 1 (Fastest) | Registers | < 1 ns | Bytes | No | Inside CPU |
| 2 | Cache (L1, L2, L3) | 1-10 ns | KB-MB | No | On/near CPU chip |
| 3 | RAM (Primary) | 10-100 ns | 4-64 GB | No (volatile) | DDR4, DDR5 |
| 4 | SSD/HDD (Secondary) | µs-ms | 256GB-10TB | Yes (non-volatile) | Hard disk, SSD |
| 5 (Slowest) | Optical/Tape (Tertiary) | seconds | TB+ | Yes | CD, DVD, tape backup |
RAM vs ROM:
| RAM (Random Access Memory) | ROM (Read-Only Memory) |
|---|---|
| Volatile — data lost when power off | Non-volatile — data stays when power off |
| Read AND write | Read only (mostly) |
| Stores currently running programs & data | Stores BIOS/firmware (boot instructions) |
| Larger (4GB-64GB) | Smaller (few MB) |
| Types: SRAM (cache), DRAM (main memory) | Types: PROM, EPROM, EEPROM |
Storage Units:
1 Bit = 0 or 1 (smallest unit)
1 Nibble = 4 bits
1 Byte = 8 bits (can represent one character)
1 KB = 1024 Bytes
1 MB = 1024 KB
1 GB = 1024 MB
1 TB = 1024 GB
1 Nibble = 4 bits
1 Byte = 8 bits (can represent one character)
1 KB = 1024 Bytes
1 MB = 1024 KB
1 GB = 1024 MB
1 TB = 1024 GB
📚 Software
Software is a set of instructions (programs) that tells the hardware what to do. Without software, hardware is useless.
| System Software | Application Software |
|---|---|
| Manages/controls the computer itself | Performs specific tasks for users |
| Runs in the background | Runs on top of system software |
| OS (Windows, Linux, macOS), Device Drivers, Compilers | MS Word, Chrome, Photoshop, Games, Excel |
Operating System Functions:
- Process Management: Runs and manages multiple programs simultaneously
- Memory Management: Allocates RAM to programs, deallocates when done
- File Management: Creates, deletes, organizes files and folders
- Device Management: Controls input/output devices via drivers
- Security: User authentication, access control, virus protection
- User Interface: GUI (graphical) or CLI (command line) for user interaction
Programming Language Levels:
| Level | Language | Example | Features |
|---|---|---|---|
| Low-level | Machine Language | 10110111 01001000 | Binary only, fastest, hardware-specific |
| Low-level | Assembly Language | MOV AX, 5 | Uses mnemonics, needs assembler |
| High-level | C, Java, Python | printf("Hello"); | Human-readable, needs compiler/interpreter |
Compiler vs Interpreter:
| Compiler | Interpreter |
|---|---|
| Translates ENTIRE program at once | Translates line by line |
| Creates executable file (.exe) | No separate file created |
| Faster execution (already compiled) | Slower execution (translates each time) |
| Shows all errors after compilation | Stops at first error |
| C, C++, Java | Python, JavaScript, Ruby |
Unit 2: Number Systems & Conversions
📚 Number Systems — How Computers Think in Numbers
Humans use decimal (base 10) because we have 10 fingers. Computers use binary (base 2) because electronics have two states: ON (1) and OFF (0). We also use hexadecimal and octal as shorthand for binary.
| System | Base | Digits Used | Example | Use |
|---|---|---|---|---|
| Binary | 2 | 0, 1 | 1010₂ = 10₁₀ | Inside computer circuits |
| Octal | 8 | 0-7 | 12₈ = 10₁₀ | Unix file permissions |
| Decimal | 10 | 0-9 | 255₁₀ | Daily human use |
| Hexadecimal | 16 | 0-9, A-F | FF₁₆ = 255₁₀ | Memory addresses, colors (#FF0000) |
Decimal to Binary Conversion:
Convert 25₁₀ to binary:
Repeatedly divide by 2 and note remainders (read bottom to top):
25 ÷ 2 = 12 remainder 1
12 ÷ 2 = 6 remainder 0
6 ÷ 2 = 3 remainder 0
3 ÷ 2 = 1 remainder 1
1 ÷ 2 = 0 remainder 1
Read bottom → top: 25₁₀ = 11001₂
Repeatedly divide by 2 and note remainders (read bottom to top):
25 ÷ 2 = 12 remainder 1
12 ÷ 2 = 6 remainder 0
6 ÷ 2 = 3 remainder 0
3 ÷ 2 = 1 remainder 1
1 ÷ 2 = 0 remainder 1
Read bottom → top: 25₁₀ = 11001₂
Binary to Decimal Conversion:
Convert 11001₂ to decimal:
Multiply each digit by 2 raised to its position (right to left, starting from 0):
1×2⁴ + 1×2³ + 0×2² + 0×2¹ + 1×2⁰
= 16 + 8 + 0 + 0 + 1 = 25₁₀
Multiply each digit by 2 raised to its position (right to left, starting from 0):
1×2⁴ + 1×2³ + 0×2² + 0×2¹ + 1×2⁰
= 16 + 8 + 0 + 0 + 1 = 25₁₀
Decimal to Hexadecimal:
Convert 255₁₀ to hex:
255 ÷ 16 = 15 remainder 15 (F)
15 ÷ 16 = 0 remainder 15 (F)
Read bottom → top: 255₁₀ = FF₁₆
255 ÷ 16 = 15 remainder 15 (F)
15 ÷ 16 = 0 remainder 15 (F)
Read bottom → top: 255₁₀ = FF₁₆
Binary Arithmetic:
Binary Addition:
0+0=0, 0+1=1, 1+0=1, 1+1=10 (0 carry 1)
1011 (11)
+ 0110 (6)
————
10001 (17) ✔
0+0=0, 0+1=1, 1+0=1, 1+1=10 (0 carry 1)
1011 (11)
+ 0110 (6)
————
10001 (17) ✔
1's and 2's Complement (for representing negative numbers):
1's Complement: Flip all bits (0→1, 1→0)
1010 → 0101
2's Complement: 1's complement + 1
1010 → 0101 + 1 = 0110
This is how computers represent negative numbers!
To find −5 in 8-bit: +5 = 00000101 → 1's = 11111010 → +1 = 11111011 = −5
1010 → 0101
2's Complement: 1's complement + 1
1010 → 0101 + 1 = 0110
This is how computers represent negative numbers!
To find −5 in 8-bit: +5 = 00000101 → 1's = 11111010 → +1 = 11111011 = −5
Unit 3: Boolean Algebra & Logic Gates
📚 Boolean Algebra
Boolean algebra works with only two values: 0 (false) and 1 (true). Named after George Boole. It's the mathematical foundation of digital circuits.
Basic Operations:
| Operation | Symbol | Meaning | Truth Table |
|---|---|---|---|
| AND | A · B or A ∧ B | Output 1 ONLY if BOTH inputs are 1 | 0·0=0, 0·1=0, 1·0=0, 1·1=1 |
| OR | A + B or A ∨ B | Output 1 if ANY input is 1 | 0+0=0, 0+1=1, 1+0=1, 1+1=1 |
| NOT | A' or ¬A | Flips the value | 0'=1, 1'=0 |
Derived Operations:
| Gate | Expression | Output |
|---|---|---|
| NAND | (A · B)' | Opposite of AND: 1,1,1,0 |
| NOR | (A + B)' | Opposite of OR: 1,0,0,0 |
| XOR | A ⊕ B | Output 1 if inputs are DIFFERENT: 0,1,1,0 |
| XNOR | (A ⊕ B)' | Output 1 if inputs are SAME: 1,0,0,1 |
NAND and NOR are called Universal Gates because ANY other gate can be built using
only NAND gates (or only NOR gates).
Boolean Laws (memorize these!):
| Law | AND Form | OR Form |
|---|---|---|
| Identity | A · 1 = A | A + 0 = A |
| Null/Domination | A · 0 = 0 | A + 1 = 1 |
| Idempotent | A · A = A | A + A = A |
| Complement | A · A' = 0 | A + A' = 1 |
| Double Negation | (A')' = A | |
| Commutative | A · B = B · A | A + B = B + A |
| Associative | (AB)C = A(BC) | (A+B)+C = A+(B+C) |
| Distributive | A(B+C) = AB + AC | A + BC = (A+B)(A+C) |
| Absorption | A(A+B) = A | A + AB = A |
| De Morgan's | (AB)' = A' + B' | (A+B)' = A' · B' |
Simplification Example:
Simplify: F = AB + AB'
= A(B + B') ← Factor out A (distributive law)
= A · 1 ← B + B' = 1 (complement law)
= A ← A · 1 = A (identity law)
Simplify: F = AB + AB'
= A(B + B') ← Factor out A (distributive law)
= A · 1 ← B + B' = 1 (complement law)
= A ← A · 1 = A (identity law)
📚 Combinational Circuits
Half Adder: Adds two single bits. Outputs: Sum (XOR) and Carry (AND).
| A | B | Sum (A⊕B) | Carry (A·B) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
Full Adder: Adds three bits (A, B, and Carry-in from previous position). Needed for multi-bit addition.
Sum = A ⊕ B ⊕ Cᵢₙ
Carry-out = AB + Cᵢₙ(A ⊕ B)
Carry-out = AB + Cᵢₙ(A ⊕ B)
Other Important Circuits:
- Multiplexer (MUX): Many inputs → one output. Selects one input based on select lines. Like a switch.
- Demultiplexer (DEMUX): One input → many outputs. Routes input to one of many output lines.
- Encoder: 2ⁿ inputs → n outputs. Converts active input to binary code.
- Decoder: n inputs → 2ⁿ outputs. Converts binary code to activate one output.
📚 Sequential Circuits & Flip-Flops
Sequential circuits have memory — output depends on current input AND previous state. This is unlike combinational circuits where output depends only on current input.
Flip-Flops (1-bit memory elements):
| Type | Inputs | Behavior | Use |
|---|---|---|---|
| SR Flip-Flop | Set, Reset | S=1: output becomes 1. R=1: output becomes 0. Both 1: invalid! | Basic memory cell |
| D Flip-Flop | Data | Output follows input on clock edge. Simplest type. | Registers, data storage |
| JK Flip-Flop | J, K | Like SR but J=K=1 toggles output (no invalid state) | Counters, most versatile |
| T Flip-Flop | Toggle | T=1: output toggles (flips). T=0: no change. | Counters |
Registers: Group of flip-flops that store multiple bits. An 8-bit register stores one byte.
Counters: Circuits that count in sequence (0,1,2,3... or 3,2,1,0...). Made from flip-flops.
Unit 4-5: System Organization & I/O
📚 Bus Architecture & I/O Systems
A bus is a set of wires that carry data between computer components. Like a highway connecting different parts of a city.
Types of Buses:
| Bus | Carries | Direction | Width Example |
|---|---|---|---|
| Data Bus | Actual data being processed | Bidirectional (both ways) | 32-bit, 64-bit |
| Address Bus | Memory addresses (WHERE to read/write) | Unidirectional (CPU → memory) | 32-bit = 4GB addressable |
| Control Bus | Control signals (read/write, clock, interrupt) | Bidirectional | Various signals |
I/O Data Transfer Methods:
| Method | How It Works | CPU Involvement | Speed |
|---|---|---|---|
| Programmed I/O | CPU continuously checks if device is ready (polling) | 100% — CPU waiting the whole time | Slowest |
| Interrupt-Driven I/O | Device sends interrupt signal when ready. CPU does other work while waiting. | Moderate — CPU works until interrupted | Medium |
| DMA (Direct Memory Access) | DMA controller transfers data directly between device and memory WITHOUT CPU | Minimal — CPU free for other tasks | Fastest |
Interrupts: Signals that temporarily halt the CPU's current work to handle a higher-priority task.
- Hardware Interrupt: From external devices (keyboard press, mouse click, printer ready)
- Software Interrupt: From programs requesting OS services (system calls)
- When interrupted: CPU saves current state → handles interrupt via ISR (Interrupt Service Routine) → returns to previous task