Encodings: ASCII, ISCII and Unicode for ISC Class 11

What Encodings is about

Weightage: Encodings is topic 2 of the ISC Class XI Computer Science syllabus, in Section A - Hardware. CISCE weights Section A at 25 of the 70 marks in Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Computer Science syllabus, examination year 2028, Class XI unit weightage

Key concepts

What a character encoding is

A character set is a collection of characters (letters, digits, punctuation, symbols, control characters). A character encoding assigns each character a unique number (code) and specifies how that number is stored in bits and bytes.

Why it is needed: computers store only binary numbers. When you type 'A', the keyboard sends a code; the program stores 01000001; the screen looks up the code and draws 'A'. If the sender and receiver use different encodings, text appears garbled (strange symbols or question marks).

Number of characters vs bits: with n bits, 2ⁿ different codes are possible.

Bits	Codes
7	128
8	256
16	65,536

Control characters (non-printing) include line feed (10), carriage return (13) and tab (9).

Worked example

How many bits are needed at minimum to give a unique code to each of 26 capital letters, 26 small letters, 10 digits and 30 symbols? [1 mark]

Try it first, then show the solutionSolution

1. Total = 26 + 26 + 10 + 30 = 92 characters; 2⁶ = 64 < 92 ≤ 128 = 2⁷.

Answer: 7 bits.

ASCII and its limitations

ASCII (American Standard Code for Information Interchange) is a 7-bit code with 128 characters (codes 0-127):

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

Useful patterns: a small letter's code = capital letter's code + 32; the digit d has code 48 + d.

Extended ASCII uses 8 bits (256 codes); codes 128-255 differ between vendors and regions, which caused incompatibility.

Limitations of ASCII:

1. Only 128 characters: enough for English, not for other languages.
2. Cannot represent Indian scripts (Devanagari, Bengali, Tamil and others), Chinese, Japanese, Arabic, or accented European letters.
3. Extended 8-bit versions are not standard: the same code means different characters in different code pages.
4. No mixing of scripts in one document reliably.
5. No mathematical and other special symbols beyond a few.

Worked example

The ASCII code of 'G' is 71. Find the codes of 'g' and 'J'. [1 mark]

Try it first, then show the solutionSolution

1. 'g' = 71 + 32 = 103; 'J' is three letters after 'G': 71 + 3 = 74.

Answer: 'g' = 103; 'J' = 74.

ISCII: Indian Script Code for Information Interchange

ISCII is an 8-bit encoding standardised by the Bureau of Indian Standards (IS 13194:1991) for Indian scripts.

Features:

1. Codes 0-127 are the same as ASCII, so English text still works.
2. Codes 128-255 represent the letters of Indian scripts (vowels, consonants, matras, halant).
3. Because the Brahmi-derived Indian scripts share a common structure, one code table serves many scripts (Devanagari, Bengali, Gurmukhi, Gujarati, Oriya, Tamil, Telugu, Kannada, Malayalam); a special script-switching code selects which script to display.
4. Supports transliteration: the same text can be displayed in another Indian script by changing the script.

Limitations: only one Indian script at a time without switching codes; not widely supported by international software; largely replaced by Unicode, whose Indian-script blocks were based on ISCII's layout.

Worked example

Why can an ISCII file show both English and Hindi text? [1 mark]

Try it first, then show the solutionSolution

1. ISCII keeps codes 0-127 identical to ASCII and uses 128-255 for Indian-script letters.

Answer: Because its lower half is ASCII (English) and its upper half holds Indian-script characters.

Unicode: one code for every character

Unicode is a universal standard that gives every character of every writing system a unique number called a code point, written U+ followed by hexadecimal digits (U+0041 = 'A'; U+0915 = 'क').

Features:

1. Covers almost all the world's scripts, plus symbols, mathematical signs and emoji.
2. The code space runs from U+0000 to U+10FFFF: 17 planes of 65,536 code points = 1,114,112 possible code points.
3. First 128 code points = ASCII, so ASCII text is valid Unicode.
4. Scripts are arranged in blocks: Devanagari U+0900-U+097F, Bengali U+0980-U+09FF, Tamil U+0B80-U+0BFF, and so on. The Devanagari digits are U+0966 (०) to U+096F (९).
5. Encoding forms store code points as bytes: UTF-8 (1 to 4 bytes per character; ASCII characters take 1 byte, Devanagari letters 3 bytes); UTF-16 (2 bytes for most characters, 4 for rare ones); UTF-32 (4 bytes each).

Unicode for the local language: a Hindi word such as कमल is stored as the code points U+0915 (क), U+092E (म), U+0932 (ल). Matras and the halant are separate code points that combine with consonants when displayed.

Advantages over ASCII and ISCII: many scripts in the same document; one standard worldwide; supported by all modern operating systems, browsers and languages.

Worked example

Suppose the word 'Namaste' (7 Latin letters) and a 6-character Devanagari word are stored in UTF-8. How many bytes does each take? [2 marks]

Try it first, then show the solutionSolution

1. Latin letters are ASCII: 1 byte each in UTF-8 → 7 bytes.
2. Devanagari characters take 3 bytes each in UTF-8 → 6 × 3 = 18 bytes.

Answer: 7 bytes and 18 bytes.

Java uses Unicode: characters and strings in the local language

In Java, the char type is a 16-bit Unicode (UTF-16) value, and String is a sequence of chars. So Java programs can store and process text in Hindi, Tamil or any script.

Key points:

1. A char can be written directly ('क') or as a Unicode escape '\u0915'.
2. Casting a char to int gives its code: (int) 'A' is 65.
3. Arithmetic on chars works on codes: (char) ('A' + 2) is 'C'.
4. String methods (length, charAt, indexOf, equals) work for any script.
5. Source files should be saved as UTF-8 and compiled with that encoding (javac -encoding UTF-8) when they contain non-ASCII letters.

The program below prints the codes of Latin characters and of the Hindi word कमल, typed directly in the UTF-8 source file (compile with javac -encoding UTF-8).

// Shows the numeric codes Java stores for characters: ASCII letters and Devanagari letters.
public class CharCodes
{
    public static void main(String[] args)
    {
        String latin = "Cab9";
        for (int i = 0; i < latin.length(); i++)
        {
            char c = latin.charAt(i);
            System.out.println(c + " -> " + (int) c);
        }
        char a = 'A';
        char next = (char) (a + 2);                 // arithmetic on codes
        System.out.println("'A' + 2 = " + next);
        System.out.println("'a' - 'A' = " + ('a' - 'A'));

        String hindi = "कमल";        // the word kamal in Devanagari
        System.out.println(hindi + " has " + hindi.length() + " chars");
        for (int i = 0; i < hindi.length(); i++)
        {
            int code = hindi.charAt(i);
            System.out.println("U+" + Integer.toHexString(code).toUpperCase() + " = " + code);
        }
    }
}

Output:

C -> 67
a -> 97
b -> 98
9 -> 57
'A' + 2 = C
'a' - 'A' = 32
कमल has 3 chars
U+915 = 2325
U+92E = 2350
U+932 = 2354

Note that each Devanagari letter is one char in Java, and its code (for example 2325 = 0x915) falls in the Devanagari block.

Worked example

What does System.out.println((char) ('k' - 32)); print, and why? [1 mark]

Try it first, then show the solutionSolution

1. 'k' has code 107; 107 − 32 = 75, which is 'K'.

Answer: K, because small and capital letters differ by 32 in ASCII and Unicode.

Table lookup for local-language equivalents

A table lookup converts Latin strings to local-language equivalents by searching a stored table.

Two techniques:

1. Parallel arrays (or a 2-D table): english[i] and hindi[i] hold matching words. Search english for the input; return hindi at the same index; if not found, return a message.
2. Code offset: when a script's characters are arranged in the same order as the Latin ones, a fixed offset converts them. Devanagari digits start at U+0966, so digit d becomes (char) ('\u0966' + d).

Algorithm (parallel arrays):

for i = 0 to n - 1
    if english[i] equals word (ignoring case)
        return hindi[i]
return "not found"

The program below converts digits by offset and day names by parallel arrays.

// Table lookup: converts Latin digits in a string to Devanagari digits, and English
// day names to Hindi using two parallel arrays.
public class LocalDigits
{
    static String toDevanagariDigits(String s)
    {
        String result = "";
        for (int i = 0; i < s.length(); i++)
        {
            char c = s.charAt(i);
            if (c >= '0' && c <= '9')
                result += (char) ('०' + (c - '0'));   // U+0966 is Devanagari zero
            else
                result += c;
        }
        return result;
    }

    static String[] english = {"Sunday", "Monday", "Tuesday"};
    static String[] hindi = {"रविवार",
                             "सोमवार",
                             "मंगलवार"};

    static String lookup(String word)
    {
        for (int i = 0; i < english.length; i++)
            if (english[i].equalsIgnoreCase(word))
                return hindi[i];
        return "?";                                       // not found in the table
    }

    public static void main(String[] args)
    {
        System.out.println(toDevanagariDigits("Class 11, Room 204"));
        System.out.println("Monday -> " + lookup("Monday"));
        System.out.println("tuesday -> " + lookup("tuesday"));
        System.out.println("Friday -> " + lookup("Friday"));
    }
}

Output:

Class ११, Room २०४
Monday -> सोमवार
tuesday -> मंगलवार
Friday -> ?

Worked example

Using the offset method, which Devanagari digit code represents 7? [1 mark]

Try it first, then show the solutionSolution

1. Code = 0x966 + 7 = 0x96D = 2413 (decimal).

Answer: U+096D (2413 in decimal), the Devanagari digit seven.

Encodings in real life

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

Topics in this chapter

• Character Encodings: ASCII, ISCII and Unicode
• Characters in Java: char, Unicode, Character Arithmetic and Table Lookup

Encodings: syntax and key facts

Codes from bits

Number of codes = 2ⁿ for n bits

7 bits: 128; 8 bits: 256; 16 bits: 65,536.

ASCII patterns

small letter = capital + 32; code of digit d = 48 + d

'A' = 65, 'a' = 97, '0' = 48.

Unicode code space

17 planes × 65,536 = 1,114,112 code points (U+0000 to U+10FFFF)

UTF-8 sizes

ASCII character = 1 byte; Devanagari character = 3 bytes; UTF-16: 2 bytes for most characters

Devanagari digit

Code of digit d = 0x966 + d

Encodings: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Encodings

The question patterns to expect, and the method that works for each.

Tricks to remember Encodings

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

• 65, 97, 48

  'A' = 65, 'a' = 97, '0' = 48: 'Sixty-five Apples, ninety-seven apples, forty-eight zeros'.

  Use it when: ASCII codes.

• ASCII ⊂ ISCII ⊂ (layout of) Unicode

  Each newer code keeps the old ASCII codes at the start.

  Use it when: Compatibility.

• U+09xx is Devanagari

  Hindi lives at U+0900 to U+097F.

  Use it when: Unicode blocks.

• Java char = 16-bit Unicode

  That is why 'क' fits in a char.

  Use it when: Java and local languages.

Encodings: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Encodings

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

• ✗Mistake: Writing that ASCII is an 8-bit code

  Standard ASCII is 7-bit (128 characters); 8-bit versions are extended ASCII.

  ✓Do this: ASCII 7 bits; extended ASCII 8 bits.

  Usually costs: 1 mark.

• ✗Mistake: Confusing Unicode with UTF-8

  Unicode assigns code points; UTF-8 is one way of storing them as bytes.

  ✓Do this: Code point (number) vs encoding form (bytes).

  Usually costs: 1 mark.

• ✗Mistake: Assuming every character is 1 byte

  In UTF-8, Devanagari letters take 3 bytes; in UTF-16, 2 bytes each.

  ✓Do this: Count bytes per encoding form.

  Usually costs: 1-2 marks.

• ✗Mistake: Forgetting the not-found case in a table lookup

  Programs must handle words missing from the table.

  ✓Do this: Return a message after the loop ends.

  Usually costs: 1 mark.

Practice questions on Encodings

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 · MCQExam level

   The Unicode block for Devanagari begins at:
   (a) U+0041
   (b) U+0900
   (c) U+0B80
   (d) U+FFFF

   Hint

   Hindi lives in U+09xx.

   Answer

   (b) U+0900.

2. Q2 · 1 mark · Very short answerExam level

   Expand ISCII.

   Hint

   Indian.

   Answer

   Indian Script Code for Information Interchange.

3. Q3 · 2 marks · Short answerExam level

   State three limitations of ASCII.

   Hint

   Size, scripts, standardisation.

   Answer

   Only 128 characters; cannot represent Indian and other non-Latin scripts or accented letters; extended 8-bit versions are not standard, so the same code means different characters on different systems.

4. Q4 · 2 marks · Short answerHarder exam level

   What does the following print? char c = 'D'; c += 3; System.out.println(c + " " + (int) c);

   Hint

   Compound assignment keeps char type.

   Answer

   G 71 ('D' is 68; 68 + 3 = 71 = 'G'; compound assignment casts back to char).

5. Q5 · 2 marks · Short answerHarder exam level

   How many bytes does a string of 50 Devanagari characters take in UTF-8 and in UTF-16? Which is smaller?

   Hint

   3 and 2 bytes.

   Answer

   UTF-8: 50 × 3 = 150 bytes; UTF-16: 50 × 2 = 100 bytes; UTF-16 is smaller for Devanagari-only text.

6. Q6 · 5 marks · Short answerHarder exam level

   Explain how Unicode represents a word in a local language, using an example.

   Hint

   Code points, block, encoding form.

   Answer

   Each letter has a unique code point in its script's block. The Hindi word कमल is the sequence U+0915 (क), U+092E (म), U+0932 (ल), all in the Devanagari block U+0900-U+097F. Matras and the halant are separate code points combined by the font when shown. The code points are stored as bytes by an encoding form: in UTF-8 each takes 3 bytes (9 bytes in all); in UTF-16, 2 bytes each. Because Java chars are UTF-16 values, a Java String holds the word directly.

7. Q7 · 5 marks · Short answerHOTS / competency

   Write a Java method int countUpper(String s) that counts capital Latin letters using character codes (without Character methods), and state its result for "ISC Class XI".

   Hint

   Codes 65 to 90.

   Answer

   int countUpper(String s) { int k = 0; for (int i = 0; i < s.length(); i++) { int c = s.charAt(i); if (c >= 65 && c <= 90) k++; } return k; }. For "ISC Class XI" the capitals are I, S, C, C, X, I: result 6.

8. Q8 · 2 marks · Short answerHOTS / competency

   Why did Unicode's Indian-script blocks follow the ISCII layout, and what advantage does Unicode have over ISCII?

   Hint

   Compatibility; mixing scripts.

   Answer

   Following ISCII's arrangement made conversion of existing Indian-language data easy. Unicode's advantage is that every script has its own code points, so many scripts can appear together in one text without switching codes, and it is supported worldwide.

9. Q9 · 2 marks · Short answerHOTS / competency

   Suppose a program converts the string "2026" to Devanagari digits using the offset 0x966. Give the code points produced and the resulting string.

   Hint

   Add the digit value to 0x966.

   Answer

   2 → U+0968, 0 → U+0966, 2 → U+0968, 6 → U+096C; the string is २०२६.

10. Q10 · 5 marks · Long answerHOTS / competency

    Write an algorithm that converts a string to capital letters using only character codes, and trace it for "aB3c".

    Hint

    Subtract 32 for 97-122.

    Answer

    result = ""; for each character c in s: if code(c) is between 97 and 122, append the character with code code(c) − 32; else append c unchanged. Return result. Trace: 'a' (97) → 'A'; 'B' (66) unchanged; '3' (51) unchanged; 'c' (99) → 'C'. Result "AB3C".

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.Bits in standard ASCII?Show answer

   7 (128 characters).

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

   65 and 97.

3. 3.ISCII bits?Show answer

   8.

4. 4.Highest Unicode code point?Show answer

   U+10FFFF.

5. 5.Bytes for a Devanagari letter in UTF-8?Show answer

   3.

6. 6.Size of a Java char?Show answer

   16 bits (UTF-16).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Encoding: character ↔ number ↔ bits.
• ☐ASCII: 7-bit, 128; 'A' 65, 'a' 97, '0' 48; English only.
• ☐ISCII: 8-bit, lower half ASCII, upper half Indian scripts (BIS 1991).
• ☐Unicode: code points U+0000-U+10FFFF; Devanagari U+0900-U+097F; UTF-8, UTF-16, UTF-32.
• ☐Java char = 16-bit Unicode; '\u0915' = क.
• ☐Lookup: parallel arrays or code offset (digits from U+0966).

Encodings: frequently asked questions