Skip to content

ASCII, ISCII and Unicode Encodings, ISC Class 11

questionbuddy.in/isc/class-11/computer-science/encodings/ascii-iscii-and-unicode-character-encodings

Computer ScienceISC Class 11 · Encodings

ASCII, ISCII and Unicode: Character Encodings (ISC Class 11 Computer Science)

Updated 17 October 2026 · Fact-checked

A character encoding assigns a unique number (code) to each character so that text can be stored and exchanged. ASCII is a 7-bit code for 128 characters (English letters, digits, symbols, controls); ISCII is an 8-bit code for Indian scripts whose lower half equals ASCII; Unicode gives every character of every writing system a unique code point (U+0000 to U+10FFFF), with ASCII as its first 128 code points.

Understand Character Encodings: ASCII, ISCII and Unicode

Encoding. A computer stores numbers, so each character is stored as a number (its code). An encoding is the agreed table of characters and codes; the same table must be used to read the text.

ASCII (American Standard Code for Information Interchange): 7 bits, 128 characters, codes 0 to 127.

RangeCharacters
0-31, 127Control characters (tab 9, line feed 10, carriage return 13)
32Space
48-57Digits '0' to '9'
65-90'A' to 'Z'
97-122'a' to 'z'

Patterns: a small letter's code = capital letter's code + 32; the digit d has code 48 + d. *Extended ASCII* (8 bits, 256 codes) differs between vendors. *Limits:* only 128 characters, enough for English only; cannot represent other scripts; the 8-bit extensions are incompatible.

ISCII (Indian Script Code for Information Interchange): 8-bit, standardised by the Bureau of Indian Standards. Codes 0-127 are ASCII; codes 128-255 carry the letters of Indian scripts. One table serves many Brahmi-derived scripts, with a code to select the script, so text can be shown in another Indian script (transliteration). *Limits:* only Indian scripts and English; cannot include other world scripts together.

Unicode. A universal standard that gives every character a unique number called a code point, written U+ followed by hexadecimal digits (U+0041 = 'A'; U+0915 = the Devanagari letter ka). Code space U+0000 to U+10FFFF: 1,114,112 code points. First 128 code points = ASCII. Scripts are in blocks (Devanagari U+0900-U+097F). Encoding forms: UTF-8 (1 to 4 bytes, ASCII characters take 1 byte), UTF-16 (2 or 4 bytes) and UTF-32 (4 bytes).

Number of codes with n bits = 2 to the power n: 7 bits give 128; 8 bits give 256; 16 bits give 65,536.

Character Encodings: ASCII, ISCII and Unicode: key facts

Codes from bits
Number of codes = 2^n | n = number of bits
7 bits: 128; 8 bits: 256.
ASCII patterns
Small letter = Capital + 32 | Digit d = 48 + d
Case conversion.
Unicode space
U+0000 to U+10FFFF = 17 x 65,536 = 1,114,112 code points | Hex to decimal: U+0915 = 2,325
Code points.

How to answer Character Encodings: ASCII, ISCII and Unicode questions

Use the code patterns and the power-of-two rule.

  1. 1Find the code of a letter from A = 65 or a = 97.
  2. 2Convert case by adding or subtracting 32.
  3. 3For storage size multiply characters by bytes per character.
  4. 4Convert hexadecimal code points to decimal when needed.

Character Encodings: ASCII, ISCII and Unicode in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Storage size

Suppose a message of 1,000 characters in English is stored once in ASCII (1 byte per character) and once in UTF-16 (2 bytes per character for these characters).

How many bytes does each need?

Try it first, then show the solutionSolution
  1. ASCII: 1,000 x 1 = 1,000 bytes.
  2. UTF-16: 1,000 x 2 = 2,000 bytes.

Answer: 1,000 bytes and 2,000 bytes.

Memory hook: More bits per character, more storage.

Tricks to remember Character Encodings: ASCII, ISCII and Unicode

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • 65, 97, 48

    A, a, 0: the anchor codes.

    Use it when: ASCII.

  • Plus 32

    Lower = upper + 32.

    Use it when: Case change.

  • Unicode = unique for all

    One code point per character, all scripts.

    Use it when: Unicode.

Character Encodings: ASCII, ISCII and Unicode: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: ASCII bits · 1 mark · MCQBoard level

The number of characters that can be represented in 7-bit ASCII is:
(a) 64 (b) 128 (c) 256 (d) 512

Try it first, then show the solutionSolution
  1. 2 to the power 7 = 128.

Answer: (b) 128

Example 2: Comparison · 4 marks · Short answerBoard level

Distinguish between ASCII and Unicode on four points. (4 marks)

Try it first, then show the solutionSolution
  1. Size: 7 bits vs code points up to 21 bits (U+10FFFF).
  2. Characters: 128 vs over a million possible.
  3. Languages: English only vs all scripts.
  4. Compatibility: ASCII is the first 128 Unicode code points.

Answer: ASCII is a 7-bit code of 128 characters for English; Unicode assigns code points to all scripts (up to 1,114,112), and its first 128 are the ASCII codes.

Example 3: Codes of letters · 5 marks · Short answerHarder board level

Find (i) the ASCII code of "Java" letter by letter and their sum; (ii) the code of the lower-case letter for 'D'; (iii) the character with code 71; (iv) the code of the digit '7'. (5 marks)

Try it first, then show the solutionSolution
  1. (i) J = 74, a = 97, v = 118, a = 97; sum = 74 + 97 + 118 + 97 = 386.
  2. (ii) 'D' = 68, so 'd' = 68 + 32 = 100.
  3. (iii) 71 - 65 = 6, the seventh letter: 'G'.
  4. (iv) 48 + 7 = 55.

Answer: (i) 74, 97, 118, 97 (sum 386); (ii) 100; (iii) G; (iv) 55.

Example 4: Storage and code points · 6 marks · Long answerHarder board level

A text of 2,500 English characters is saved in ASCII (1 byte each) and UTF-16 (2 bytes each), and a text of 600 Devanagari characters is saved in UTF-16 (2 bytes each) and in UTF-8 (3 bytes each for this block). Find the sizes. Convert U+0915 to decimal. How many Devanagari block code points are there from U+0900 to U+097F? (6 marks)

Try it first, then show the solutionSolution
  1. English: ASCII 2,500 bytes; UTF-16 5,000 bytes.
  2. Devanagari: UTF-16 1,200 bytes; UTF-8 1,800 bytes.
  3. U+0915 = 9 x 256 + 1 x 16 + 5 = 2,304 + 16 + 5 = 2,325.
  4. Block size = 0x97F - 0x900 + 1 = 127 + 1 = 128 code points.

Answer: 2,500 and 5,000 bytes; 1,200 and 1,800 bytes; U+0915 = 2,325; the Devanagari block has 128 code points.

Example 5: Why Unicode over ISCII and ASCII · 5 marks · Long answerHOTS / competency

A news website must display English, Hindi, Tamil and Japanese text on one page. Explain, with three points, why ASCII and ISCII are unsuitable and Unicode is suitable, and say what error appears if a Unicode page is read with the wrong encoding. (5 marks)

Try it first, then show the solutionSolution
  1. ASCII has only English characters.
  2. ISCII covers Indian scripts and English but not Japanese.
  3. Unicode has code points for all four scripts in one standard.
  4. Reading with the wrong encoding gives garbled characters (mojibake) or question marks because the same bytes map to different characters.

Answer: ASCII lacks other scripts and ISCII lacks Japanese; Unicode covers all in one table; a wrong encoding shows garbled text.

Where students lose marks in Character Encodings: ASCII, ISCII and Unicode

The slips examiners see most often, and what to write instead.

  • Mistake: Saying ASCII has 256 characters

    Extended ASCII confusion.

    Wrong working

    ASCII = 256

    Right working

    Standard ASCII = 128 (7 bits)

    Do this: Distinguish extended ASCII.

    Usually costs: 1 mark.

  • Mistake: Adding 32 to turn a small letter into a capital

    Wrong direction.

    Wrong working

    'a' + 32 = 129

    Right working

    'a' - 32 = 65 = 'A'

    Do this: Small letters have the larger codes.

    Usually costs: 1 mark.

  • Mistake: Mixing U+0915 hex with decimal

    Base slip.

    Wrong working

    U+0915 = 915

    Right working

    2,325

    Do this: Convert from hex.

    Usually costs: 1-2 marks.

Exam tips

  • Anchor codes: A = 65, a = 97, 0 = 48.
  • Mention that Unicode includes ASCII as its first 128 code points.
  • Give the formula 2^n for the number of codes.

Practice questions on Character Encodings: ASCII, ISCII and Unicode

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    The ASCII code of the character 'a' is:
    (a) 65 (b) 97 (c) 48 (d) 122

    Hint

    Small letters begin at 97.

    Answer

    (b) 97

  2. Q2 · 3 marks · Short answerBoard level

    State two limitations of ASCII and how Unicode overcomes them. (3 marks)

    Hint

    128 characters; English only.

    Answer

    ASCII has only 128 characters and cannot represent other scripts. Unicode gives a unique code point to every character of every script (over a million code points).

  3. Q3 · 4 marks · Short answerHarder board level

    Find the character with ASCII code 109 and the codes of 'Z' and 'z'. A string "ABC" is converted to lower case by adding 32 to each code: write the codes before and after. (4 marks)

    Hint

    109 - 97 = 12.

    Answer

    109 is 'm'. 'Z' = 90 and 'z' = 122. "ABC": 65, 66, 67 become 97, 98, 99 ("abc").

  4. Q4 · 5 marks · Long answerHarder board level

    How many distinct codes can a 10-bit code hold, and how many bits are needed at least to give unique codes to the 1,114,112 Unicode code points? A file of 4,000 Hindi characters is stored in UTF-8 (3 bytes each) and in UTF-32 (4 bytes each); compare. (5 marks)

    Hint

    2^10 and 2^20 = 1,048,576.

    Answer

    10 bits give 1,024 codes. Since 2^20 = 1,048,576 < 1,114,112 <= 2^21 = 2,097,152, at least 21 bits are needed. UTF-8: 12,000 bytes; UTF-32: 16,000 bytes; UTF-8 saves 4,000 bytes.

  5. Q5 · 3 marks · Short answerHOTS / competency

    A school record system stored in ISCII is to be merged with a Unicode database that contains names in Bengali, Tamil and Chinese. Explain the compatibility problem and a safe migration step. (3 marks)

    Hint

    Different tables.

    Answer

    ISCII codes 128-255 mean different characters from Unicode code points, so a direct merge would produce wrong characters, and ISCII cannot hold Chinese. The records should be converted from ISCII to Unicode with a conversion table before merging.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.ASCII bits?Show answer

    7 (128 characters).

  2. 2.Code of 'A'?Show answer

    65.

  3. 3.Unicode code space?Show answer

    U+0000 to U+10FFFF.

  4. 4.ISCII lower half equals?Show answer

    ASCII.

  5. 5.Small letter code vs capital?Show answer

    Plus 32.

Character Encodings: ASCII, ISCII and Unicode: frequently asked questions

Is UTF-8 same as Unicode?

No, UTF-8 is an encoding form of Unicode.

Who standardised ISCII?

The Bureau of Indian Standards.

Is ASCII valid in Unicode?

Yes, as its first 128 code points.

What is a code point?

The unique number of a character.