CS111 Foundations

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CS111
Notes
Spring 2009

The ASCII Character Set

We have now seen how numbers are represented in the computer. We use two's complement for integers and floating point for reals.

A text document is broken into individual characters. There are a finite number of characters to represent. So the approach is to list them all and assign each a binary string. To store a particular letter, we store the appropriate bit string.

What characters do we worry about? Upper and lower case letters, punctuation, numeric digits, white space, special symbols. What symbols would we need if we were not using English?

A character set is a list of characters and the codes used to represent them. Several codes have been used over the years. IBM created a code for its mainframe computers. It is the Extended Binary Coded Decimal Information Code (EBCDIC). It is an 8-bit code. Then we got American Standard Code for Information Interchange (ASCII), used by PC's. It began as an 7-bit code with the extra bit used for a variety of purposes including checking that the code was transmitted correctly. But 7 bits allow only 128 characters. This is fine if everyone communicates in English, but a few other languages exist. The 7-bit code was extended to be an 8-bit code which allows 256 different characters. Now other languages can be included: Spanish, German, French and most other European languages. The third system in the Unicode Character Set which uses 16 bits per character. Now we can express over 65.000 characters. More languages can be represented, including Russian, Thai, Greek, Cherokee, Braille, Chinese/Japanese/Korean, Mathematical symbols, and special symbols such as ₤ and ™. In future versions even Klingon will be supported.

Below is the table of printable and nonprintable ASCII characters.

Nonprintable ASCII Characters			Printable ASCII Characters
Decimal	Hex	Name	Decimal	Hex	Name
0	0	NULL	32	20	(Space)
1	01	SOTT	33	21	!
2	02	STX	34	22	"
3	03	ETY	35	23	#
4	04	EOT	36	24	$
5	05	ENQ	37	25	%
6	06	ACK	38	26	&
7	07	BELL	39	27	'
8	08	BKSPC	40	28	(
9	09	HZTAB	41	29	)
10	0A	NEWLN	42	2A	*
11	0B	VTAB	43	2B	+
12	0C	FF	44	2C	,
13	0D	CR	45	2D	-
14	0E	CO	46	2E	.
15	0F	SI	47	2F	/
16	10	DLE	48	30	0
17	11	DC1	49	31	1
18	12	DC2	50	32	2
19	13	DC3	51	33	3
20	14	DC4	52	34	4
21	15	NAK	53	35	5
22	16	SYN	54	36	6
23	17	ETB	55	37	7
24	18	CAN	56	38	8
25	19	EM	57	39	9
26	1A	SUB	58	3A	:
27	1B	ESC	59	3B	;
28	1C	FS	60	3C	>
29	1D	GS	61	3D	=
30	1E	RS	62	3E	<
31	1F	US	63	3F	?
127	7F	DEL	64	40	@
			65	41	A
			66	42	B
			67	43	C
			68	44	D
			69	45	E
			70	46	F
			71	47	G
			72	48	H
			73	49	I
			74	4A	J
			75	4B	K
			76	4C	L
			77	4D	M
			78	4E	N
			79	4F	O
			80	50	P
			81	51	Q
			82	52	R
			83	53	S
			84	54	T
			85	55	U
			86	56	V
			87	57	W
			88	58	X
			89	59	Y
			90	5A	Z
			91	5B	[
			92	5C	\
			93	5D	]
			94	5E	^
			95	5F	_
			96	60	`
			97	61	a
			98	62	b
			99	63	c
			100	64	d
			101	65	e
			102	66	f
			103	67	g
			104	68	h
			105	69	i
			106	6A	j
			107	6B	k
			108	6C	l
			109	6D	m
			110	6E	n
			111	6F	o
			112	70	p
			113	71	q
			114	72	r
			115	73	s
			116	74	t
			117	75	u
			118	76	v
			119	77	w
			120	78	x
			121	79	y
			122	7A	z
			123	7B	{
			124	7C	\|
			125	7D	}
			126	7E	~

Computers can layer encodings to virtually any level of complexity. Numbers can be interpreted as characters, which can be interpreted in sets as Web pages, which can be interpreted to appear as multiple fonts and styles. But at the bottommost level, the computer only "knows" voltages, which we interpret as numbers.

Syllabus

Notes

Revised - 8 January 2009