What is ASCII and how many characters does it define?

ASCII (American Standard Code for Information Interchange) defines 128 characters: 32 non-printable control characters (0–31), 95 printable characters including letters, digits, and symbols (32–126), and one DEL character (127). It was standardised in 1963 and remains the foundation of modern text encoding — every UTF-8 byte below 128 is identical to its ASCII value.

What are ASCII control characters and what do they do?

Control characters (codes 0–31) are non-printable characters that once controlled printers and terminals. Common ones still used today: NUL (0) — null terminator in C strings; TAB (9) — horizontal tab; LF (10) — line feed / newline on Unix; CR (13) — carriage return on Windows (CRLF line endings); ESC (27) — escape sequences in terminals and ANSI codes; DEL (127) — delete.

Why does ASCII use decimal 65 for the letter A?

The assignment was a deliberate design choice. Uppercase letters start at 65 (0x41), lowercase at 97 (0x61). The difference between any uppercase and its lowercase counterpart is exactly 32 — a single bit (bit 5) — making case conversion trivially cheap in hardware. Digits start at 48 (0x30), so the numeric value of digit n is its ASCII code minus 48.

ASCII Table Reference | CodeLint.Dev

128 characters

Dec	Hex	Oct	Binary	Char	Abbr	Description	HTML
0	0x00	0000	00000000	—	NUL	Null
1	0x01	0001	00000001	—	SOH	Start of Heading
2	0x02	0002	00000010	—	STX	Start of Text
3	0x03	0003	00000011	—	ETX	End of Text
4	0x04	0004	00000100	—	EOT	End of Transmission
5	0x05	0005	00000101	—	ENQ	Enquiry
6	0x06	0006	00000110	—	ACK	Acknowledge
7	0x07	0007	00000111	—	BEL	Bell
8	0x08	0010	00001000	—	BS	Backspace
9	0x09	0011	00001001	—	HT	Horizontal Tab
10	0x0A	0012	00001010	—	LF	Line Feed
11	0x0B	0013	00001011	—	VT	Vertical Tab
12	0x0C	0014	00001100	—	FF	Form Feed
13	0x0D	0015	00001101	—	CR	Carriage Return
14	0x0E	0016	00001110	—	SO	Shift Out
15	0x0F	0017	00001111	—	SI	Shift In
16	0x10	0020	00010000	—	DLE	Data Link Escape
17	0x11	0021	00010001	—	DC1	Device Control 1 (XON)
18	0x12	0022	00010010	—	DC2	Device Control 2
19	0x13	0023	00010011	—	DC3	Device Control 3 (XOFF)
20	0x14	0024	00010100	—	DC4	Device Control 4
21	0x15	0025	00010101	—	NAK	Negative Acknowledge
22	0x16	0026	00010110	—	SYN	Synchronous Idle
23	0x17	0027	00010111	—	ETB	End of Transmission Block
24	0x18	0030	00011000	—	CAN	Cancel
25	0x19	0031	00011001	—	EM	End of Medium
26	0x1A	0032	00011010	—	SUB	Substitute
27	0x1B	0033	00011011	—	ESC	Escape
28	0x1C	0034	00011100	—	FS	File Separator
29	0x1D	0035	00011101	—	GS	Group Separator
30	0x1E	0036	00011110	—	RS	Record Separator
31	0x1F	0037	00011111	—	US	Unit Separator
32	0x20	0040	00100000
33	0x21	0041	00100001	!		!
34	0x22	0042	00100010	"		"	"
35	0x23	0043	00100011	#		#
36	0x24	0044	00100100	$		$
37	0x25	0045	00100101	%		%
38	0x26	0046	00100110	&		&	&
39	0x27	0047	00100111	'		'
40	0x28	0050	00101000	(		(
41	0x29	0051	00101001	)		)
42	0x2A	0052	00101010	*		*
43	0x2B	0053	00101011	+		+
44	0x2C	0054	00101100	,		,
45	0x2D	0055	00101101	-		-
46	0x2E	0056	00101110	.		.
47	0x2F	0057	00101111	/		/
48	0x30	0060	00110000	0		0
49	0x31	0061	00110001	1		1
50	0x32	0062	00110010	2		2
51	0x33	0063	00110011	3		3
52	0x34	0064	00110100	4		4
53	0x35	0065	00110101	5		5
54	0x36	0066	00110110	6		6
55	0x37	0067	00110111	7		7
56	0x38	0070	00111000	8		8
57	0x39	0071	00111001	9		9
58	0x3A	0072	00111010	:		:
59	0x3B	0073	00111011	;		;
60	0x3C	0074	00111100	<		<	<
61	0x3D	0075	00111101	=		=
62	0x3E	0076	00111110	>		>	>
63	0x3F	0077	00111111	?		?
64	0x40	0100	01000000	@		@
65	0x41	0101	01000001	A		A
66	0x42	0102	01000010	B		B
67	0x43	0103	01000011	C		C
68	0x44	0104	01000100	D		D
69	0x45	0105	01000101	E		E
70	0x46	0106	01000110	F		F
71	0x47	0107	01000111	G		G
72	0x48	0110	01001000	H		H
73	0x49	0111	01001001	I		I
74	0x4A	0112	01001010	J		J
75	0x4B	0113	01001011	K		K
76	0x4C	0114	01001100	L		L
77	0x4D	0115	01001101	M		M
78	0x4E	0116	01001110	N		N
79	0x4F	0117	01001111	O		O
80	0x50	0120	01010000	P		P
81	0x51	0121	01010001	Q		Q
82	0x52	0122	01010010	R		R
83	0x53	0123	01010011	S		S
84	0x54	0124	01010100	T		T
85	0x55	0125	01010101	U		U
86	0x56	0126	01010110	V		V
87	0x57	0127	01010111	W		W
88	0x58	0130	01011000	X		X
89	0x59	0131	01011001	Y		Y
90	0x5A	0132	01011010	Z		Z
91	0x5B	0133	01011011	[		[
92	0x5C	0134	01011100	\		\
93	0x5D	0135	01011101	]		]
94	0x5E	0136	01011110	^		^
95	0x5F	0137	01011111	_		_
96	0x60	0140	01100000	`		`
97	0x61	0141	01100001	a		a
98	0x62	0142	01100010	b		b
99	0x63	0143	01100011	c		c
100	0x64	0144	01100100	d		d
101	0x65	0145	01100101	e		e
102	0x66	0146	01100110	f		f
103	0x67	0147	01100111	g		g
104	0x68	0150	01101000	h		h
105	0x69	0151	01101001	i		i
106	0x6A	0152	01101010	j		j
107	0x6B	0153	01101011	k		k
108	0x6C	0154	01101100	l		l
109	0x6D	0155	01101101	m		m
110	0x6E	0156	01101110	n		n
111	0x6F	0157	01101111	o		o
112	0x70	0160	01110000	p		p
113	0x71	0161	01110001	q		q
114	0x72	0162	01110010	r		r
115	0x73	0163	01110011	s		s
116	0x74	0164	01110100	t		t
117	0x75	0165	01110101	u		u
118	0x76	0166	01110110	v		v
119	0x77	0167	01110111	w		w
120	0x78	0170	01111000	x		x
121	0x79	0171	01111001	y		y
122	0x7A	0172	01111010	z		z
123	0x7B	0173	01111011	{		{
124	0x7C	0174	01111100	\|		\|
125	0x7D	0175	01111101	}		}
126	0x7E	0176	01111110	~		~
127	0x7F	0177	01111111	—	DEL	Delete

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