Why Computers See 'A' as 65: The ASCII Encoding Guide

What Is ASCII?

ASCII (American Standard Code for Information Interchange) assigns a numeric code to each English character. For example:

'A' → 65

'B' → 66

'a' → 97

'0' → 48

Space → 32

Computers store the binary representation of the code, not the symbol itself. The letter "A" is stored as 01000001 (binary for 65). When reading, the computer sees 01000001, converts to 65, looks up the ASCII table, and displays "A". Encoding = character to number (storage); decoding = number to character (display).

ASCII Table Overview

ASCII defines 128 characters (codes 0-127), fitting in 7 bits.

Control Characters (0-31 and 127)

These are non-printable codes used to control devices:

0 (NUL) – Null character

7 (BEL) – Bell (makes terminal beep)

8 (BS) – Backspace

9 (TAB) – Horizontal tab

10 (LF) – Line feed (\n)

13 (CR) – Carriage return (\r)

27 (ESC) – Escape

127 (DEL) – Delete

Printable Characters (32-126)

Visible characters grouped by code ranges:

32 – Space

33 – !

34 – "

40-41 – ( )

42-47 – * + , - . /

48-57 – Digits 0-9

65-90 – Uppercase A-Z

91-96 – [ ] ^ _ `

97-122 – Lowercase a-z

123-126 – { | } ~

Key ASCII Codes to Remember

'A' = 65 (uppercase letters start at 65)

'a' = 97 (lowercase letters start at 97)

'0' = 48 (digit characters start at 48)

Space = 32

Newline (\n) = 10

Important pattern: Uppercase 'A' (65) and lowercase 'a' (97) differ by 32. Therefore case conversion is simply adding or subtracting 32. Both uppercase and lowercase letters are contiguous sequences.

Extended ASCII (128-255)

Standard ASCII uses only 7 bits; the 8th bit in a byte was later used to define codes 128-255 for extra symbols and European accented characters. Examples:

130 → é (French e with acute)

169 → © (copyright sign)

176 → ° (degree sign)

Different vendors and countries assigned different meanings to the same extended codes, causing incompatibility. This chaos was one reason Unicode was created.

ASCII in Programming

Character Comparison

Comparing characters compares their ASCII values:

'A' < 'B'  → 65 < 66 → true
'a' > 'A'  → 97 > 65 → true
'0' < '9'  → 48 < 57 → true

Character and Integer Conversion (C)

char c = 'A';  // c holds 65
int n = c + 1; // n becomes 66, which is 'B'

Case Conversion

char c = 'A';        // 65
char lower = c + 32; // 97, which is 'a'

Limitations of ASCII

ASCII only supports English. It lacks:

Chinese characters

Japanese, Korean scripts

Arabic, Cyrillic scripts

Chinese punctuation (。 ， ！)

Before Unicode, Chinese systems used multi-byte encodings like GB2312 and GBK, which extended ASCII by using two bytes per Chinese character.

Fun Fact: ASCII 42

Code 42 is the asterisk *. In Douglas Adams' The Hitchhiker's Guide to the Galaxy , 42 is the "Answer to the Ultimate Question of Life, the Universe, and Everything" – a deliberate programmer joke.

Conclusion

ASCII is the foundational character encoding: computers don't understand characters, only numbers. Every character is encoded into a number for storage. Grasping this concept makes learning Unicode, UTF-8, and GBK much easier.