Computer Systems & Organization – Complete Notes for KVS/NVS/EMRS PGT CS

1. Author’s Note: The Foundation of Your Success

By Sriram

“Welcome, aspirants.

In my teaching Computer Science and clearing competitive exams like UGC NET and SET, I have noticed a recurring pattern: Students often fail not because they don’t know complex programming, but because they overlook the basics. They can write Python code but cannot explain how a Compiler is different from an Interpreter, or how the CPU actually fetches an instruction.

This module is not just a ‘chapter’ in your syllabus; it is the grammar of Computer Science. In exams like KVS and NVS, this single unit often carries 15-20% of the weightage. Questions here are binary—you either know the logic gate truth table, or you don’t. There is no guessing.

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2. The Genesis: Basic Computer Organization

We often define a computer as an electronic device, but for a Computer Science student, that definition is insufficient. A computer is a programmable machine that manipulates data according to a list of instructions.

To understand how it works, we must look at the Von Neumann Architecture, the model proposed by physicist John von Neumann in 1945, which still powers almost every computer today.

2.1 The “IPOS” Cycle: The Universal Rhythm

Every computing task, whether it is calculating a missile trajectory or playing Candy Crush, follows the IPOS cycle.

1. Input (I): The raw data provided to the system.
   • Devices: Keyboard, Mouse, Scanner, Microphone.
   • Sriram’s Insight: Input is not just ‘typing’. Even the temperature sensor in a smart AC is an input device providing raw data.
2. Process (P): The manipulation of data to transform it into information.
   • The Engine: This happens inside the Central Processing Unit (CPU).
3. Output (O): The processed information presented to the user.
   • Devices: Monitor (Soft Copy), Printer (Hard Copy), Speakers.
4. Storage (S): Saving the data for future use.
   • Types: Immediate (RAM) vs. Permanent (HDD).

The “Kitchen” Analogy: Whenever I teach this topic, I use the analogy of a Restaurant Kitchen to explain the components:

• Input: The Waiter brings the raw order ticket (Data).
  • Technical Equivalent: Keyboard/Mouse.
• CPU (The Head Chef): The person who actually chops, cooks, and plates the food. They do the work.
  • Technical Equivalent: Microprocessor.
• RAM (The Kitchen Counter): The Chef cannot run to the fridge for every onion. He keeps current ingredients on the counter. It is fast, close, but small.
  • Technical Equivalent: Random Access Memory.
• Hard Disk (The Deep Freezer): The huge storage room in the back where tons of ingredients are stored. It takes time to walk there, but it holds everything.
  • Technical Equivalent: Secondary Storage.
• Output: The Waiter brings the cooked dish to your table.
  • Technical Equivalent: Monitor.

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3. Inside the Brain: The Central Processing Unit (CPU)

The CPU is often called the brain, but technically, it is more like the heart and brain combined. It pumps information and makes decisions.

3.1 The Three Pillars of the CPU

A modern CPU is not a single block; it is composed of three distinct functional units.

A. The Arithmetic Logic Unit (ALU)

This is the “Workhorse” of the computer. It performs two types of operations:

1. Arithmetic Operations: Addition, Subtraction, Multiplication, Division.
2. Logical Operations: Comparisons (Greater Than, Less Than, Equal To) and Boolean Logic (AND, OR, NOT).
   • Example: If you ask the computer “Is \(5 > 3 \)?”, the ALU evaluates this and returns “TRUE”.

B. The Control Unit (CU)

This is the “Manager” or “Traffic Police” of the system.

• Crucial Concept: The CU does not process data. It never adds numbers or stores files.
• Function: It fetches instructions from memory, decodes them, and generates Control Signals to tell the other units (ALU, Memory, I/O) what to do.
• Sriram’s Exam Tip: If a question asks “Which unit initiates the fetching of instructions?”, the answer is Control Unit.

C. Registers (The CPU’s Personal Notepad)

Registers are high-speed storage locations directly inside the CPU. They are the fastest memory in the computer hierarchy.

• Accumulator (ACC): Stores the intermediate results of calculations.
• Program Counter (PC): Stores the address of the next instruction to be executed.
• Memory Address Register (MAR): Holds the address of the active memory location.
• Instruction Register (IR): Holds the current instruction being executed.

3.2 The System Bus: The Nervous System

How do these components talk to each other? Through wires called Buses.

1. Data Bus: Carries the actual data (e.g., the number “45”) between CPU and Memory. It is Bi-directional.
2. Address Bus: Carries the address (location) of where data should be read/written. It is Uni-directional (CPU to Memory).
3. Control Bus: Carries commands (Read, Write, Interrupt) from the CU.

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4. Memory Hierarchy: The Speed vs. Cost Trade-off

One of the most important concepts for the PGT exam is the Memory Hierarchy. Computers need memory that is fast (to keep up with the CPU) and large (to store movies/games). But fast memory is expensive. So, we organize memory in layers.

4.1 Level 0: CPU Registers

• Location: Inside the CPU core.
• Speed: Instant (Nanoseconds).
• Capacity: Tiny (64 bits or 128 bits).

4.2 Level 1: Cache Memory (SRAM)

Cache solves the speed mismatch between the super-fast CPU and the slower RAM. It stores frequently used data.

• L1 Cache: Smallest and fastest, built into the CPU.
• L2/L3 Cache: Larger but slightly slower.
• Technology: Uses SRAM (Static RAM) which is faster and more expensive than the DRAM used in main memory.

4.3 Level 2: Primary Memory (RAM & ROM)

Also known as Main Memory. The CPU can access this directly.

A. RAM (Random Access Memory):

• Volatile: Data is lost when power is turned off.
• Read/Write: You can read data and write new data.
• Types:
  • DRAM (Dynamic RAM): Needs to be refreshed thousands of times per second. Slower, cheaper. Used in your laptop (e.g., 8GB DDR4).
  • SRAM (Static RAM): Does not need refreshing. Faster, expensive. Used in Cache.

B. ROM (Read Only Memory):

• Non-Volatile: Retains data even without power.
• Read Only: Cannot be easily overwritten.
• Purpose: Stores the BIOS/UEFI (Bootstrap Loader)—the first program that runs when you turn on the PC to check hardware and load the OS.

4.4 Level 3: Secondary Storage

Since RAM is volatile and expensive, we need permanent storage.

1. HDD (Hard Disk Drive): Uses spinning magnetic platters. Cheaper, high capacity, but fragile and slow.
2. SSD (Solid State Drive): Uses Flash Memory (like USB drives). No moving parts. Much faster and durable than HDD, but more expensive per GB.

Sriram’s Calculation Trap: Question: You have a 1 TB Hard Disk. How many Bytes is that?

• Student Mistake: \(10^{12} \) Bytes (Decimal definition).
• CS Definition: \(2^{40} \) Bytes.
• Always look at the options. If they use powers of 2, stick to the binary definition (\(1024 \) GB).

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5. Software Concepts: The Ghost in the Machine

If Hardware is the body, Software is the soul. Without software, a computer is just a pile of silicon and metal.

5.1 System Software

This creates the platform for other software to run. It interacts directly with hardware.

1. Operating System (OS): The resource manager (Windows, Linux, Android).
2. Device Drivers: Small programs that teach the OS how to communicate with specific hardware (e.g., Graphics Driver, Printer Driver).
3. Language Translators: Compilers, Interpreters, Assemblers.

5.2 Application Software

Designed for end-users to perform specific tasks.

• General Purpose: MS Word, Chrome, Photoshop.
• Customized (Bespoke): Software made for a specific client (e.g., The software used by ISRO for a specific mission, or a School Management System).

5.3 Language Processors: The Translators

(Crucial Topic for PGT) Computers speak Binary (Machine Code). Humans speak High-Level Languages (Python, C++, Java). We need a translator to bridge this gap.

Feature	Compiler	Interpreter
Input	Scans the entire program at once.	Translates line-by-line.
Output	Generates an Object Code (.exe) file.	Does not save object code; runs directly.
Speed	Execution is Faster (once compiled).	Execution is Slower (translates every time).
Errors	Shows all errors after scanning the whole code.	Stops execution at the first error found.
Examples	C, C++, Java.	Python, Ruby, JavaScript.

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Sriram’s Insight: Why is Python popular despite being slower (Interpreter)? Because it is easier to debug. If your code crashes on line 50, an interpreter stops exactly there, letting you fix it instantly. A compiler would throw 100 errors at once, which is scary for beginners!

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6. The Operating System (OS): The Manager

The OS is the most important program that runs on a computer. It manages the computer’s memory and processes, as well as all of its software and hardware.

6.1 Core Functions of an OS

1. Process Management: The OS decides which program gets access to the CPU and for how long. (e.g., Listening to Spotify while typing in Word).
2. Memory Management: Allocates RAM to different apps. If RAM is full, it uses Virtual Memory (borrowing space from the HDD).
3. File Management: Keeps track of where files are stored (FAT32, NTFS, EXT4 systems).
4. Device Management: Handles inputs/outputs using drivers.

6.2 Types of Operating Systems

1. Single User, Single Task: Can do only one thing at a time. (e.g., MS-DOS).
2. Multi-User, Multi-Tasking: Modern OSs (Windows, Linux, macOS).
3. Real-Time OS (RTOS): Used where strict time deadlines are mandatory. (e.g., Air Traffic Control, Missile Guidance). Even a microsecond delay can be fatal.
4. Time-Sharing OS: The CPU switches between multiple users/tasks so fast that it gives the illusion of simultaneous execution.

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7. Boolean Logic: The Mathematics of Digital Circuits

This section is pure logic. In digital electronics, there are no “maybes”. There is only True (1/High/On) and False (0/Low/Off).

7.1 Logic Gates

A Logic Gate is a physical device implementing a Boolean function.

1. AND Gate:

• Logic: “If both A and B are True, then Output is True.”
• Symbol: \(\cdot \) (Dot).
• Truth Table:
  • 0 AND 0 = 0
  • 0 AND 1 = 0
  • 1 AND 0 = 0
  • 1 AND 1 = 1

2. OR Gate:

• Logic: “If either A or B is True, then Output is True.”
• Symbol: \(+ \) (Plus).
• Truth Table:
  • 0 OR 0 = 0
  • 0 OR 1 = 1
  • 1 OR 0 = 1
  • 1 OR 1 = 1

3. NOT Gate (Inverter):

• Logic: “Reverse the input.”
• Symbol: \(\bar{A} \) (Bar) or \(A’ \).
• Truth Table:
  • NOT 0 = 1
  • NOT 1 = 0

4. NAND Gate (Universal Gate):

• Logic: NOT of AND.
• Output: 0 only if both inputs are 1.

5. NOR Gate (Universal Gate):

• Logic: NOT of OR.
• Output: 1 only if both inputs are 0.

7.2 De Morgan’s Laws (The Golden Rules)

These laws are used to simplify complex boolean expressions. You will see a question on this.

• First Law: The complement of a sum is equal to the product of the complements.
  • \(\overline{A + B} = \bar{A} \cdot \bar{B} \)
  • Mnemonic: “Break the line, change the sign” (Change OR to AND).
• Second Law: The complement of a product is equal to the sum of the complements.
  • \(\overline{A \cdot B} = \bar{A} + \bar{B} \)
  • Mnemonic: “Break the line, change the sign” (Change AND to OR).

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8. Number Systems: The Language of Computers

We humans use the Decimal System (Base 10) because we have 10 fingers. Computers use Binary (Base 2) because they are made of transistors that have two states (On/Off).

8.1 The Four Number Systems

System	Base	Symbols	Used In
Binary	2	0, 1	Machine Language
Octal	8	0-7	Unix Permissions
Decimal	10	0-9	Human Counting
Hexadecimal	16	0-9, A-F	Color Codes, Memory Addresses

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(Note on Hex: A=10, B=11, C=12, D=13, E=14, F=15)

8.2 Master Conversion Guide

Case 1: Decimal to Any Base (Divide & Remainder)

Method: Repeatedly divide the decimal number by the target base and record remainders bottom-to-top.

Example: Convert Decimal \(25 \) to Binary.

1. \(25 \div 2 = 12 \), Remainder 1
2. \(12 \div 2 = 6 \), Remainder 0
3. \(6 \div 2 = 3 \), Remainder 0
4. \(3 \div 2 = 1 \), Remainder 1
5. \(1 \div 2 = 0 \), Remainder 1

• Result: Read bottom-up \(\rightarrow (11001)_2 \).

Case 2: Any Base to Decimal (Power of Base)

Method: Multiply each digit by the base raised to its position power (starting from 0 on the right).

Example: Convert Binary \(1011 \) to Decimal.

• \((1 \times 2^3) + (0 \times 2^2) + (1 \times 2^1) + (1 \times 2^0) \)
• \(8 + 0 + 2 + 1 = 11 \)
• Result: \(11 \).

Case 3: Binary to Octal/Hex (The Grouping Shortcut)

This is the shortcut I teach my students to save time in exams.

• Binary to Octal: Group bits in 3s from right to left.
  • Question: \(110101 \)
  • Group: \((110) \) \((101) \)
  • Convert: \(6 \) \(5 \)
  • Result: \((65)_8 \)
• Binary to Hex: Group bits in 4s from right to left.
  • Question: \(11111010 \)
  • Group: \((1111) \) \((1010) \)
  • Convert: \(15(F) \) \(10(A) \)
  • Result: \((FA)_{16} \)

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9. Encoding Schemes: Representing Text

How does a computer understand the letter ‘A’ or the symbol ‘@’? It uses Encoding Schemes to map characters to numbers.

1. ASCII (American Standard Code for Information Interchange):
   • The oldest standard. Uses 7 bits.
   • Can represent 128 characters (\(2^7 \)).
   • Limitation: Only supports English.
2. ISCII (Indian Script Code for Information Interchange):
   • Developed by the Indian Government to support Indian languages (Hindi, Telugu, Tamil, etc.).
   • Uses 8 bits (\(2^8 = 256 \) characters).
3. Unicode (The Global Standard):
   • Uses variable bits (8, 16, or 32).
   • Can represent every written language in the world, plus symbols and emojis.
   • UTF-8 is the most popular version used on the web today.

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10. Exam Corner: Sriram’s High-Yield Questions

These questions are curated from previous KVS/NVS/NET papers.

Q1: Which component of the CPU acts as the “Traffic Policeman”?

• A) ALU
• B) CU
• C) Registers
• D) Bus
• Answer: B (Control Unit). It directs flow, doesn’t process data.

Q2: What is the 2’s complement of the binary number 1010?

• Method: Invert digits (\(0101 \)) and add 1 (\(0101 + 1 \)).
• Answer: \(0110 \).

Q3: Which of the following is Non-Volatile Memory?

• A) SRAM
• B) DRAM
• C) Cache
• D) EEPROM
• Answer: D (EEPROM). All RAM types are volatile. ROM types (like EEPROM) are non-volatile.

Q4: A 4-bit number can represent a maximum decimal value of?

• Formula: \(2^n – 1 \)
• Calc: \(2^4 – 1 = 16 – 1 = 15 \).
• Answer: 15.

Q5: Which gate returns a High output (1) only when inputs are different?

• A) OR
• B) NAND
• C) XOR
• D) NOR
• Answer: C (XOR). Exclusive OR checks for difference.

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11. Summary & Next Steps

Congratulations! You have just completed a deep dive into the architecture of the computer. We covered everything from the IPOS cycle to the complex logic of De Morgan’s laws.

Key Takeaways for the Exam:

1. Hierarchy: Registers > Cache > RAM > HDD.
2. Logic: NAND and NOR are Universal Gates.
3. Math: Practice Binary \(\leftrightarrow \) Decimal conversions until you can do them mentally.
4. Software: Know the difference between Compiler and Interpreter.

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