Appendix A.3: ASCII

ASCII. (American Standard Code for Information Interchange) is an encoding format for text developed in the early 1960’s.

The original ASCII format was based on the English alphabet and encodes 128 specified characters into seven-bit binary numbers.

Ninety-five of the encoded characters are printable, including the digits 0 to 9, lowercase letters a to z, uppercase letters A to Z, space, and punctuation symbols.

The remaining 32 non-printing control codes for based around the standards original implementation with Teletype machines – most of these are now obsolete but some are still used including carriage return (\r in C), line feed (\n in C), tab (\t in C).

As microprocessors evolved to 8-bit and higher the ASCII standard has also evolved to use the eighth bit allowing a further 127 characters (extended ASCII).[1]

Table 20 ANSI ASCII Table

Dec	Hex	Symbol	Dec	Hex	Symbol	Dec	Hex	Symbol	Dec	Hex	Symbol
0	0x00	Null character[2]	32	0x20	Space	64	0x40	@	96	0x60	`
1	0x01	Start of heading	33	0x21	!	65	0x41	A	97	0x61	a
2	0x02	Start of text	34	0x22	“	66	0x42	B	98	0x62	b
3	0x03	End of text	35	0x23	#	67	0x43	C	99	0x63	c
4	0x04	End of Transmission	36	0x24	$	67	0x44	D	100	0x64	d
5	0x05	Enquiry	37	0x25	%	64	0x45	E	101	0x65	e
6	0x06	Acknowledgement	38	0x26	&	64	0x46	F	102	0x66	f
7	0x07	Bell	39	0x27	‘	64	0x47	G	103	0x67	g
8	0x08	Backspace	40	0x08	(	64	0x48	H	104	0x68	h
9	0x09	Horizontal tab	41	0x29	)	64	0x49	I	105	0x69	i
10	0x0A	Line feed	42	0x2A		64	0x4A	J	106	0x6A	j
11	0x0B	Vertical tab	43	0x2B	+	64	0x4B	K	107	0x6B	k
12	0x0C	Form feed	44	0x2C	,	64	0x4C	L	108	0x6C	l
13	0x0D	Carriage return	45	0x2D	-	64	0x4D	M	109	0x6D	m
14	0x0E	Shift out	46	0x2E	.	64	0x4E	N	110	0x6E	n
15	0x0F	Shift in	47	0x2F	/	64	0x4F	O	111	0x6F	o
16	0x10	Data link escape	48	0x30	0	65	0x50	P	112	0x70	p
17	0x11	Device control 1	49	0x31	1	64	0x51	Q	113	0x71	q
18	0x02	Device control 2	50	0x32	2	64	0x52	R	114	0x72	r
19	0x13	Device control 3	51	0x33	3	64	0x53	S	115	0x73	s
20	0x14	Device control 4	52	0x34	4	64	0x54	T	116	0x74	t
21	0x15	Negative acknowledgement	53	0x35	5	64	0x55	U	117	0x75	u
22	0x16	Synchronous idle	54	0x36	6	64	0x56	V	118	0x76	v
23	0x17	End of transmission block	55	0x37	7	64	0x57	W	119	0x77	w
24	0x18	Cancel	56	0x38	8	64	0x58	X	120	0x78	x
25	0x19	End of medium	57	0x39	9	64	0x59	Y	121	0x79	y
26	0x1A	Substitute	58	0x3A	:	64	0x5A	Z	122	0x7A	z
27	0x1B	Escape	59	0x3B	;	64	0x5B	[	123	0x7B	{
28	0x1C	File separator	60	0x3C	<	64	0x5C	\	124	0x7C	|
29	0x1D	Group separator	61	0x3D	=	64	0x4D	]	125	0x7D	}
30	0x1E	Record separator	62	0x3E	>	64	0x5E	^	126	0x7E	~
31	0x1F	Unit separator	63	0x3F	?	95	0x5F	_	127	0x7F	Delete

A very useful and useful reference is available as ascii-code.com. See also ASCII Code Chart.

Example

Encode the string “Welcome to Swansea University” in ASCII and give the binary codes that would be used to store this string in computer memory.

Solution

Look up the characters and write down the equivalent hexadecimal code. Note, null (\0) is used to terminate the string

'W' = \(87_{10}\) = 0x57 = \(01010111_2\)

'e' = \(101_{10}\) = 0x65 = \(01100101_2\)

'l' = \(108_{10}\) = 0x6C = \(01101011_2\)

'c' = \(99_{10}\) = 0x63 = \(01100011_2\)

'o' = \(111_{10}\) = 0x6F = \(01101111_2\)

'm' = \(109_{10}\) = 0x6D = \(01101101_2\)

'e' = \(108_{10}\) = 0x6C = \(01101011_2\)

' ' = \(108_{10}\) = 0x6C = \(01101011_2\)

't' = \(108_{10}\) = 0x6C = \(01101011_2\)

'o' = \(108_{10}\) = 0x6C = \(01101011_2\)

' ' = \(32_{10}\) = 0x20 = \(00100000_2\)

'S' = \(83_{10}\) = 0x53 = \(01010011_2\)

'w' = \(119_{10}\) = 0x77 = \(01110111_2\)

'a' = \(97_{10}\) = 0x61 = \(01100001_2\)

'n' = \(110_{10}\) = 0x6E = \(01101110_2\)

's' = \(115_{10}\) = 0x73 = \(01110011_2\)

'e' = \(108_{10}\) = 0x6C = \(01101011_2\)

'a' = \(97_{10}\) = 0x61 = \(01100001_2\)

\0 = 0x000 = \(00000000_2\)

The final result (in hexadecimal) is

57
65
6C
63
6F
6D
65
32
74
6E
32
53
77
61
6E
73
65
61
00

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[1]

As ASCII was only designed to represent English, extended ASCII was developed been used to extend the coding so that displays could print accented European characters, some Greek symbols used in mathematics, and some symbols that could be used to draw boxes on a simple display screen. In order to support the rest of the human languages and alphabets, and for other purposes such as Emojis, ASCII has been extended to a standard called UTF-8. This uses more bytes to represent each character and therefore greatly extends the types of texts that can be stored and manipulated inside a computer.

[2]

the null character (\0 in C) is used in C to mark the end of a string.