AS · Practice questions · Character sets: ASCII, extended ASCII and Unicode

Text, encoded.

Six original Cambridge-style questions on character sets and character codes, with worked solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

One code per character.

ASCII: 7 bits, 128 characters; extended ASCII 8 bits, 256 characters.
Unicode: up to 32 bits, every language and emoji; first 128 codes match ASCII.
Codes are consecutive: count along from the code you are given; lower case = upper case + 32 in ASCII.
Storage: number of characters × bits per character.

Explain

[2 marks]

Explain what is meant by a character set.

The set of all the characters that a computer can represent / recognise. ✓
Each character is given a unique binary code. ✓

Apply

[2 marks]

The ASCII code for 'M' is 77. Give the 8-bit binary codes for 'P' and for 'm'.

'P' = 80 = 01010000. ✓
'm' = 77 + 32 = 109 = 01101101. ✓

Apply

[3 marks]

The code for the character '0' is 48. (a) Give the 8-bit code for the character '5'. (b) Explain why this is different from storing the integer 5.

(a) 48 + 5 = 53 = 00110101. ✓
(b) The integer 5 is stored as 00000101. ✓
The character code represents a symbol for display; it must be converted before it can be used in calculations. ✓

Calculate

[3 marks]

A document contains 6000 characters. Calculate the storage needed in KiB if it is stored (a) in extended ASCII and (b) in a Unicode encoding using 4 bytes per character.

(a) 6000 bytes ÷ 1024 = 5.86 KiB (2 d.p.). ✓
(b) 6000 × 4 = 24 000 bytes. ✓
24 000 ÷ 1024 = 23.44 KiB (2 d.p.). ✓

Explain

[3 marks]

Describe the differences between ASCII and Unicode.

ASCII uses 7 bits per character; Unicode uses up to 32 bits (e.g. 16 or 32). ✓
ASCII represents only 128 characters (English letters, digits, punctuation); Unicode represents characters from all languages and emoji. ✓
Unicode text can take more storage; the first 128 Unicode codes are the same as ASCII. ✓

Analysis

[2 marks]

A company changes its customer database from extended ASCII to a Unicode encoding. Give one benefit and one drawback of the change.

Benefit: names and addresses in any language (e.g. Arabic, Chinese) can be stored correctly. ✓
Drawback: more bits per character, so the database needs more storage and takes longer to transmit. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.