Encoding Schemes and Number System Class 11 Computer Science Chapter 2 CBSE Notes 2025-26

Computers do not understand letters or words the way humans do. Instead, every key you press on a keyboard is given a unique number or code that the computer translates into binary (0s and 1s). For example, pressing the key ‘A’ generates a decimal value of 65, which is then turned into the binary number 1000001 so a computer can work with it.

Encoding Schemes and Why They Matter Standard encoding schemes were created to ensure that all computers understand these codes in the same way, regardless of brand or type. The most widely used encoding scheme is ASCII (American Standard Code for Information Interchange), which gives a unique code to each English character, number, and symbol. ASCII uses 7 bits, meaning it can represent 128 different characters commonly found on a keyboard.

Other schemes like ISCII (Indian Script Code for Information Interchange) help encode Indian language scripts. ISCII is an 8-bit system which supports up to 256 characters, allowing representation of many Indian alphabets while keeping the standard ASCII set intact. You’ll also find UNICODE. Unlike ASCII or ISCII, Unicode is a global standard designed to contain every symbol from all languages, supporting universal data exchange and communication. Unicode uses different encodings, like UTF-8, UTF-16, and UTF-32, and is a superset of ASCII—with the first 128 codes identical.

Example: Encoding Letters and Words The word DATA, for example, is encoded using ASCII as follows: ‘D’ is 68, ‘A’ is 65, ‘T’ is 84, and ‘A’ is 65. In binary (7-bit representation): D=1000100, A=1000001, T=1010100, A=1000001. This conversion ensures the computer reads and stores your input as electronic signals.

Number Systems in Computers Computers use number systems to process and store data. The decimal system (base-10) is what humans use every day, involving the digits 0 to 9. However, computers use the binary system (base-2), which involves only two digits: 0 and 1. Each digit represents an ON or OFF state in tiny electronic switches called transistors.

Binary numbers can get very lengthy as numbers increase, so other number systems like octal (base-8, digits 0-7) and hexadecimal (base-16, digits 0-9 and A-F) are often used for more compact representation. Octal groups binary digits in sets of 3, and hexadecimal groups them in sets of 4. This makes reading, writing, and debugging easier for humans.

Conversions between Number Systems To communicate efficiently, both humans and computers must convert numbers between systems. The basic method to convert decimal to binary, octal, or hexadecimal involves repeatedly dividing the decimal number by the new system’s base and writing the remainders in reverse order. For decimals with fractional parts, keep multiplying the fraction by the base and using the whole numbers at each step.

For example, to convert (65)₁₀ to binary, keep dividing by 2: you get the binary number 1000001. Similarly, (65)₁₀ in octal is 101, and in hexadecimal, it is 41. The same rule goes the other way—converting from binary, octal, or hex to decimal means multiplying each digit by its positional base value and adding them all together.

Quick Reference: Conversion Steps

• Decimal to other systems: Divide by base, note remainders, reverse them.
• Other systems to decimal: Multiply each digit by base^position, sum up.
• Fraction conversion: Multiply fraction by base, collect the integer part for each step.
• Binary to octal: Group 3 bits; to hex: group 4 bits.

Applications of Hexadecimal and Octal Systems Programmers use hexadecimal and octal systems to simplify long binary strings, especially for memory addresses or color codes in computer graphics. For instance, a 16-bit memory address in binary like 1100000011110001 would be C0F1 in hexadecimal. Colors on web pages use hex codes like #FF0000 for bright red, making it easier to handle millions of color shades compared to using binary alone.

Conversion Examples for Practice

• (1101)₂ = 1×8 + 1×4 + 0×2 + 1×1 = 13 in decimal.
• (257)₈ = 2×64 + 5×8 + 7×1 = 175 in decimal.
• (3A5)₁₆ = 3×256 + 10×16 + 5 = 933 in decimal.
• (10101100)₂ to octal: group as 010, 101, 100 ⇒ 2, 5, 4 ⇒ 254₈.
• Fractional part: (0.25)₁₀ × 2 = 0.50 (0), then ×2 = 1.00 (1) ⇒ (0.01)₂.

Summary of Key Points

• Encoding transforms keyboard input into codes (ASCII, ISCII, Unicode) computers can process.
• Computers use binary numbers for all data; octal and hexadecimal simplify binary codes for humans.
• Unicode is needed for content in all languages and supports universal text exchange.
• Conversion between number systems is essential for programming and understanding digital data.

For practice, you should try converting numbers and words between decimal, binary, octal, and hexadecimal, and explore how text is converted into codes using ASCII and Unicode—for both English and Indian languages.

CBSE Class 11 Computer Science Chapter 2 Notes – Encoding Schemes and Number System

Boost your preparation for exams with these concise CBSE Class 11 Computer Science Chapter 2 notes covering Encoding Schemes and Number System. This page provides clear explanations of binary, octal, decimal, and hexadecimal conversions, plus encoding standards like ASCII and Unicode.

Revise important tables, step-wise conversion methods, and practical applications quickly. These notes help you grasp key concepts and ensure you can answer both theory and numericals efficiently in your Class 11 Computer Science exam.