ASCII & Hex

Part of the Fundamentals section of Coddy's Assembly journey — lesson 13 of 45.

Computers only understand numbers. So how do they store letters like 'A', 'b', or '?'?

The answer is ASCII (American Standard Code for Information Interchange). ASCII is a table that assigns a number to every character.

How ASCII works:

Character	ASCII number (decimal)	ASCII number (hex)
'A'	65	`0x41`
'B'	66	`0x42`
'C'	67	`0x43`
'a'	97	`0x61`
'b'	98	`0x62`
'0'	48	`0x30`
'1'	49	`0x31`
' ' (space)	32	`0x20`
'\n' (newline)	10	`0x0A`
'\0' (null)	0	`0x00`

Important ASCII values to memorize for assembly:

Character	Hex	Why you need it
'A'	`0x41`	Uppercase letters start here
'a'	`0x61`	Lowercase letters start here
'0'	`0x30`	Digits start here
Space	`0x20`	Separating words
Newline (`\n`)	`0x0A`	Moving to next line
Null (`\0`)	`0x00`	End of string marker

Notice the pattern:

Pattern	Example
Uppercase A-Z are consecutive	'A'=65, 'B'=66, 'C'=67...
Lowercase a-z are consecutive	'a'=97, 'b'=98, 'c'=99...
Digits 0-9 are consecutive	'0'=48, '1'=49, '2'=50...
Lowercase = uppercase + 32	'a' (97) = 'A' (65) + 32

How a string is stored in memory:

The word "Hi" with a newline is stored as:

Character	Hex	Binary
'H'	`0x48`	01001000
'i'	`0x69`	01101001
'\n'	`0x0A`	00001010

In assembly, you write this as: db 'Hi', 0x0A

Why you need this for assembly:

When you define a string in data.asm, you are storing ASCII codes in memory. When you print a string, the operating system reads those ASCII codes and displays the matching characters.

Try it yourself

This lesson doesn't include a code challenge.