ASCII and Unicode: Character Encoding

Computers process only binary — every character, whether a letter, digit, or punctuation mark, must be stored as a number. Character encoding is the system that assigns a unique number (a code point) to each character, allowing software to consistently translate between binary values and readable text.

Without an agreed standard, text created on one system would display as gibberish on another. A file of English text is just a sequence of bytes; the encoding standard tells the software which character each byte represents.

Two encoding standards are required by the AQA GCSE spec:

Standard	Bits per character	Characters covered
ASCII (7-bit)	7 (stored in 8)	128 characters (0–127)
Unicode	Variable	Over 140,000 characters

A character encoding is a lookup table: a number maps to a character, and that number is what the computer stores in binary.

ASCII (American Standard Code for Information Interchange) uses 7 bits to represent each character, giving $2^7=128$ unique code points numbered 0–127.

The 128 positions are divided into three groups:

Range (denary)	Contents
0–31	Control characters — non-printable: newline, tab, backspace
32–126	Printable characters — letters, digits, punctuation, space
127	Delete (control character)

Key code points to know:

Character	ASCII code (denary)
Space	32
0	48
9	57
A	65
Z	90
a	97
z	122

In practice, ASCII is stored in 8-bit bytes with the leading bit set to 0. The standard itself is 7-bit — this distinction appears in mark schemes.

Exam questions provide an ASCII table and ask you to convert in both directions. The key reference points from the previous slide are enough to answer most questions without memorising every value.

Worked example 1 — encode Cat as ASCII code points (denary):

Character	ASCII code
C	67
a	97
t	116

Worked example 2 — decode the ASCII sequence 72, 101, 120 to text:

Code	Character
72	H
101	e
120	x

Result: Hex

ASCII stores one character per byte. A 10-character string in ASCII occupies exactly 10 bytes — no more, no less.