Character sets (ASCII, Extended ASCII, Unicode), codepoints, and how text becomes binary. Seven stages from theory to mastery. Click any stage card to dive in — or continue where you left off.
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How text becomes binary through character sets. Master ASCII (7-bit, 128 characters), Extended ASCII (8-bit, 256), and Unicode (multi-language, more bits). Cambridge IGCSE 0478 Paper 1.
Computers only store binary. So how do they store the letter 'A' or the character '?' or the word "Hello"? Answer: every character has a numeric code defined in a character set, and the code is stored in binary.
The character set is a shared codebook — sender and receiver both use the same one, so binary → back to text works reliably. Cambridge want you to know the two main character sets: ASCII (small, English-focused, 7-bit) and Unicode (huge, multi-language, more bits per character).
Note: images are covered in 1.3B Image Representation Lab and sound in 1.3C Sound Representation Lab. This lesson (1.3A) covers text only.
| Term | Definition |
|---|---|
| Character | A single letter, digit, punctuation mark, symbol, or control code that can be displayed or processed. |
| Character set | A defined collection of characters, each with a unique numeric code, that a computer can represent. |
| Codepoint / character code | The unique number assigned to a specific character in a character set (e.g. A = 65). |
| ASCII | American Standard Code for Information Interchange. Uses 7 bits per character → 128 possible codes. |
| Extended ASCII | Adds an 8th bit for a total of 8 bits per character → 256 possible codes (adds symbols, accented letters). |
| Unicode | A universal character set covering all world scripts. Uses more bits per character than ASCII (commonly 16 or 32). |
| Sequential ordering | Consecutive characters in ASCII have consecutive codes — A=65, B=66, C=67; a=97, b=98, c=99; 0=48, 1=49. |
| String | A sequence of characters, each stored as its own code. |
| Character set | Bits per character | Total characters | Coverage |
|---|---|---|---|
| ASCII | 7 | 128 | English letters (A–Z, a–z), digits (0–9), punctuation, control codes |
| Extended ASCII | 8 | 256 | ASCII + additional symbols, accented characters, graphics |
| Unicode | 16 (or 32) | 65,536 (or ~4.3 billion) | Every world script — Greek, Chinese, Arabic, Devanagari, emoji… |
| Character | Denary code | Binary (7-bit) |
|---|---|---|
| A (uppercase) | 65 | 1000001 |
| B | 66 | 1000010 |
| Z | 90 | 1011010 |
| a (lowercase) | 97 | 1100001 |
| b | 98 | 1100010 |
| z | 122 | 1111010 |
| 0 (digit) | 48 | 0110000 |
| 9 | 57 | 0111001 |
| space | 32 | 0100000 |
Consequence: to find the code of any letter, count from the anchor. K is the 11th letter (A=1, B=2… K=11) so K = 65 + 10 = 75.
| Quantity | Formula | Example |
|---|---|---|
| File size in bits | characters × bits-per-character | 50 chars × 7 = 350 bits (ASCII) |
| File size in bytes | bits ÷ 8 | 350 ÷ 8 = 43.75 → 44 bytes |
| File size doubles if… | you switch from 8-bit to 16-bit | 50 chars × 16 = 800 bits = 100 bytes |
Give the ASCII denary code for the character D.
Give the ASCII code for the character d (lowercase).
A message contains 200 characters. Calculate the file size in bytes (a) using ASCII, and (b) using 16-bit Unicode.
If Cambridge ask for the advantage of Unicode over ASCII, saying "Unicode uses more bits" is a difference, not an advantage. The advantage is what that difference enables — "represents more characters / more languages / all world scripts". Same trap in reverse for disadvantages.
Cited: 2024 Nov examiner report — "some candidates just gave a difference between the two, for example, they stated that Unicode uses more bits than ASCII but did not give the advantage of this"'A' and 'a' are two different characters with two different codes. A = 65, a = 97. The difference is exactly 32 (0100000 in binary — the sixth bit flipped). Never treat them as interchangeable.
Cited: recurring patternThe character '5' has ASCII code 53, not 5. The characters '0' to '9' are stored using their character codes (48 to 57), not the numeric value. If you want the number 5 you store it as binary 101; if you want the digit character '5' you store 53.
Cited: recurring patternASCII is defined as 7 bits per character (128 codes). Extended ASCII is 8 bits (256). Cambridge accept 7 or 8 for ASCII in questions where 8-bit storage is assumed, but be clear on which you're using.
Cited: 2023 examiner report — "some candidates gave an answer in different quantities of bits (for example 7 bits) which was acceptable"Unicode is the character set (the codebook). UTF-8, UTF-16, UTF-32 are encoding schemes that store Unicode codes in bytes. For Cambridge, just say Unicode uses more bits per character than ASCII.
Cited: syllabus-focus clarificationUnicode's main disadvantage in Cambridge questions is larger file size for the same text. If ASCII uses 8 bits and Unicode 16, the file doubles in size. State this explicitly, not as "more bits".
Cited: 2024 Nov examiner report — same trap as advantagesThese are the exact phrasings Cambridge accept. Learn them.
Answer these in your head, then tap to see the model answer. Aim for 5 or 6 right before moving on.
A shared codebook where every character (letter, digit, symbol) has a unique number.
The letter 'A' has code 65. Store 65 in binary (7 or 8 bits). Reader looks up 65 in the same codebook → gets 'A' back.
ASCII: small, 128 chars, 7 bits. Unicode: huge, all world scripts, 16+ bits.
Type a word (or a single character). Each character is looked up in ASCII and shown with its denary code and 8-bit binary.
Notice the sequential ordering: adjacent letters have adjacent codes. And case matters: 'A' = 65, 'a' = 97 (exactly 32 apart).
Type "5" above. Its ASCII code is 53, not 5. The digit characters '0'–'9' use codes 48–57.
Cited: recurring patternSame pattern for lowercase (a = 97) and digits ('0' = 48). Learn the three anchors — 65 · 97 · 48 — and you can find any code by counting.
A single character set covering every world script — Greek, Chinese, Arabic, Cyrillic, Devanagari, emoji, mathematical symbols. Uses more bits per character than ASCII (typically 16, up to 32).
Advantage: represents more characters / more languages / all world scripts.
Disadvantage: file size doubles compared to 8-bit ASCII for the same text.
"Unicode uses more bits" is a difference. To get the advantage mark, state what that enables: "more characters / more languages / all world scripts". Same for disadvantage: not "more bits", but "larger file size".
Cited: 2024 Nov examiner report — verbatimFile size (bits) = number of characters × bits per character
File size (bytes) = file size in bits ÷ 8
A 200-character message stored using 8-bit ASCII:
200 × 8 = 1600 bits = 200 bytes
The same message stored using 16-bit Unicode:
200 × 16 = 3200 bits = 400 bytes (twice as large)
A student writes: "Unicode uses more bits than ASCII" as an advantage of Unicode. Is this a valid advantage?
Given a character, type its ASCII denary code.
Press New Question.
Stretch: 5 in a row across uppercase, lowercase, and digits.
Given a short word, type the space-separated ASCII codes for each character.
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Stretch: keep case in mind — 'H' and 'h' have different codes.
Which statement about character sets is WRONG?
Press New Question.
Stretch: explain out loud why the wrong statement is wrong, using mark-scheme phrasing.
Given the character count and encoding, compute the file size.
Press New Question.
Stretch: can you predict the byte count before submitting?
Answer as many text-representation questions as you can in 60 seconds.
Press Start Sprint to begin.
Stretch: beat your personal best twice in a session.
Weak skills come round twice as often. No question repeats immediately. 20 questions across 8 skills.
Press New Question.
12 questions in Cambridge Paper 1 style. Every question cites its source. Write your answer, then click Mark for keyword scoring.
Press New Question to begin.
Press New Question.
Every trap below is drawn from a real Cambridge examiner report or a recurring pattern flagged across multiple sessions.
"Unicode uses more bits" is a difference. For advantage, say "more characters / languages / world scripts". For disadvantage, say "larger file size".
Cited: 2024 Nov verbatim — "some candidates just gave a difference between the two, for example, they stated that Unicode uses more bits than ASCII but did not give the advantage of this"A = 65, a = 97. Different characters, different codes. Difference is 32.
Cited: recurring patternThe character '5' has ASCII code 53, not 5. Digits '0' to '9' use codes 48 to 57.
Cited: recurring patternASCII defined as 7 bits (128 codes). Extended ASCII: 8 bits (256). Cambridge accept either for ASCII when 8-bit storage is assumed.
Cited: 2023 verbatim — "some candidates gave an answer in different quantities of bits (for example 7 bits) which was acceptable"Unicode is the character set (codebook). UTF-8/16/32 are encoding schemes. For Cambridge just say Unicode uses more bits per character than ASCII.
Cited: syllabus clarificationUnicode's main disadvantage in Cambridge questions is larger file size for the same text. Say "larger file size", not "more bits".
Cited: 2024 Nov examiner focusIf asked to give an example of a character set, name one: ASCII (or Unicode). Don't just describe what a character set is.
Cited: 2024 verbatim — "the most common correct answer that was awarded was a correct example of a character set, for example ASCII"Read the question. If it asks for bytes, divide bits by 8. If it asks for bits, don't divide. Wrong unit = zero marks.
Cited: recurring exam-technique flagIf A=65 and D is 4 letters later, D = 68 not 69. Count the gap from the anchor (A to D is +3, not +4).
Cited: recurring counting-slip patternA=65 · a=97 · '0'=48. Learn these three anchors and you can find any letter or digit code by counting.
A=65 → a=97 (diff 32). B=66 → b=98 (diff 32). Always +32 for the case shift.
Not "more bits" (that's a difference). Say what having more bits gives you: more characters / languages / world scripts.
The character '5' is stored as its ASCII code (53), not as the value 5. Numeric value and character code are different things.
File size formula. Multiply characters by bits-per-character to get bits. Divide by 8 for bytes.
Unicode is the codebook (character set). UTF-8/16 are encodings (how the codes are stored). For Cambridge, just say Unicode uses more bits per character.
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