SF Prep
09 Computer Awareness — Common Notes 🕒 Updated 2026-10-17
Common Notes SSC CGL · CHSL · Police · Bank · Railway Chapter 09

🔢 Number System, Codes & Logic Gates

Binary, octal, decimal, hexadecimal; conversion methods, binary addition, complements, ASCII-Unicode, logic gates and Boolean algebra.

📌 1. Number Systems

SystemBaseDigits
Binary20, 1
Octal80–7
Decimal100–9
Hexadecimal160–9, A(10)–F(15)

Conversion methods

  • Decimal → binary: divide by 2 repeatedly and read remainders from bottom to top. (25)10 = (11001)2.
  • Binary → decimal: multiply by place values 2⁰, 2¹, 2²… and add. (1011)2 = 8+0+2+1 = 11.
  • Binary → octal: groups of 3 bits from the right; binary → hex: groups of 4 bits. (101101)2 = (55)8 = (2D)16.
  • (17)8 = 1×8+7 = 15; (1F)16 = 1×16+15 = 31.
DecimalBinaryOctalHex
81000108
10101012A
12110014C
15111117F
16100002010
⚠️ Exam Traps:
  • Octal has no digit 8 or 9; (19)8 is invalid.
  • In hex F = 15 and A = 10.

📌 2. Binary Arithmetic, Complements & Codes

AdditionResult
0+00
0+11
1+10, carry 1 (=10)
1+1+11, carry 1 (=11)
  • Example: 1011 + 0110 = 10001 (11 + 6 = 17).
  • 1's complement = invert every bit (0↔1): 1010 → 0101.
  • 2's complement = 1's complement + 1: 0101 → 1010 + 1 = 1011. Computers represent negative numbers this way.
CodeFact
ASCIIAmerican Standard Code for Information Interchange — 7-bit (128 characters); extended 8-bit (256). A = 65, a = 97, 0 = 48
UnicodeAll the world's scripts (including Hindi); UTF-8, UTF-16
EBCDIC8-bit, IBM mainframes
BCDEach decimal digit in 4 bits
⚠️ Exam Traps:
  • ASCII is 7-bit; Hindi/Devanagari needs Unicode.

📌 3. Logic Gates & Boolean Algebra

GateOutput is 1 whenExpression
ANDAll inputs are 1A·B
ORAny input is 1A+B
NOTInput is 0A′
NANDNot all inputs are 1 (inverse of AND)(A·B)′
NORAll inputs are 0(A+B)′
XORInputs are differentA⊕B
XNORInputs are the same(A⊕B)′
  • NAND and NOR are universal gates — any gate can be built from them.
  • De Morgan: (A·B)′ = A′+B′ and (A+B)′ = A′·B′.
  • Boolean algebra — George Boole. A+A = A, A·A = A, A+1 = 1, A·0 = 0.
⚠️ Exam Traps:
  • Universal gates = NAND, NOR (not XOR).
  • In Boolean algebra 1 + 1 = 1 (OR); in binary addition 1 + 1 = 10.

📌 4. Practice MCQs

📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. The base of the binary number system is: Practice
(A) 8
(B) 2
(C) 10
(D) 16
Correct answer: (B)
Binary uses only 0 and 1.
Q2. How many digits/symbols are there in the hexadecimal system? Practice
(A) 8
(B) 10
(C) 16
(D) 15
Correct answer: (C)
0–9 and A–F = 16.
Q3. The binary form of (25)10 is: Practice
(A) 10011
(B) 10101
(C) 11011
(D) 11001
Correct answer: (D)
16+8+1 = 25 → 11001.
Q4. The decimal value of (1011)2 is: Practice
(A) 11
(B) 10
(C) 9
(D) 13
Correct answer: (A)
8+0+2+1 = 11.
Q5. The decimal value of (1F)16 is: Practice
(A) 25
(B) 31
(C) 15
(D) 30
Correct answer: (B)
1×16 + 15 = 31.
Q6. The decimal value of (17)8 is: Practice
(A) 13
(B) 17
(C) 15
(D) 16
Correct answer: (C)
1×8 + 7 = 15.
Q7. The hexadecimal form of (101101)2 is: Practice
(A) 3D
(B) 5B
(C) B2
(D) 2D
Correct answer: (D)
4-bit groups: 0010 1101 = 2 D.
Q8. The octal form of (101101)2 is: Practice
(A) 55
(B) 45
(C) 65
(D) 54
Correct answer: (A)
3-bit groups: 101 101 = 5 5.
Q9. Which of these is not a valid octal number? Practice
(A) 17
(B) 19
(C) 75
(D) 60
Correct answer: (B)
Octal uses digits 0–7 only; 9 is not allowed.
Q10. Binary addition 1011 + 0110 = ? Practice
(A) 11001
(B) 10101
(C) 10001
(D) 10011
Correct answer: (C)
11 + 6 = 17 = 10001.
Q11. The 2's complement of 0101 is: Practice
(A) 1010
(B) 1101
(C) 0110
(D) 1011
Correct answer: (D)
1's complement = 1010; +1 = 1011.
Q12. The 1's complement of 1010 is: Practice
(A) 0101
(B) 1011
(C) 0110
(D) 1010
Correct answer: (A)
Invert every bit.
Q13. Standard ASCII is a code of how many bits? Practice
(A) 6
(B) 7
(C) 8
(D) 16
Correct answer: (B)
7-bit = 128 characters; extended ASCII = 8-bit.
Q14. In ASCII, the decimal value of capital "A" is: Practice
(A) 48
(B) 97
(C) 65
(D) 64
Correct answer: (C)
A = 65, a = 97, digit 0 = 48.
Q15. The coding system that represents all scripts of the world, including Hindi, is: Practice
(A) ASCII
(B) EBCDIC
(C) BCD
(D) Unicode
Correct answer: (D)
Unicode (UTF-8/16) assigns codes to characters of all languages.
Q16. Which gates are called "universal gates"? Practice
(A) NAND, NOR
(B) AND, OR
(C) XOR, XNOR
(D) NOT, AND
Correct answer: (A)
Any logic circuit can be built using only NAND or only NOR.
Q17. Which gate gives output 1 only when the two inputs are different? Practice
(A) AND
(B) XOR
(C) OR
(D) XNOR
Correct answer: (B)
XOR = exclusive OR.
Q18. When is the output of an AND gate 1? Practice
(A) Any one input is 1
(B) All inputs are 0
(C) All inputs are 1
(D) Inputs differ
Correct answer: (C)
AND = multiplication; any 0 makes the output 0.
Q19. By De Morgan's law, (A·B)′ = ? Practice
(A) A′·B′
(B) A·B
(C) A+B
(D) A′+B′
Correct answer: (D)
(A·B)′ = A′ + B′ and (A+B)′ = A′·B′.
Q20. Who developed Boolean algebra? Practice
(A) George Boole
(B) Charles Babbage
(C) Claude Shannon
(D) Alan Turing
Correct answer: (A)
George Boole (19th century). Claude Shannon later applied it to circuits.

📝 Practice questions

Computer Awareness — Practice MCQs (Bilingual) Ch 9: Number System & Logic Gates | संख्या पद्धति
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