Character Encodings

Character encodings are based on a similar principle to those "unbreakable" codes kids use to communicate: A is 1, B is 2, and so forth. Messages are just sequences of numbers; to send CAT we send 3 1 20.

Computer character sets are just agreed-upon encodings like this. Back in the early days, there were a vast number of encodings; typically every manufacturer had their own; the widths of the encodings varied as well. CDC, who had a 60 bit word size, used a six bit encoding (so they could put ten characters in a word). One of the most famous manufacturer-specific encodings was IBM's Extended Binary-Coded Decimal Interchange Code.

ASCII

The American Standard Code for Information Interchange (ASCII) was proposed in 1963 and standardized in 1968. ASCII is a seven-bit code, so it can represent 128 different symbols of various types. Here's an ASCII code chart:

	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0	NUL	SOH	STX	ETX	EOT	ENQ	ACK	BEL	BS	HT	LF	VT	FF	CR	SO	SI
1	DLE	DC1	DC2	DC3	DC4	NAK	SYN	ETB	CAN	EM	SUB	ESC	FS	GS	RS	US
2	SP	!	"	#	$	%	&	'	(	)	*	+	,	-	.	/
3	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
4	@	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
5	P	Q	R	S	T	U	V	W	X	Y	Z	[	\	]	^	_
6	`	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o
7	p	q	r	s	t	u	v	w	x	y	z	{	\|	}	~	DEL

Looking at it, we can see it is divided into several sections:

All the upper case letters occur together, and all the lower case letters also occur together (EBCDIC actually had gaps!)
All the upper case letters are lexigraphically ordered, as are the lower. Also you can convert from upper case to lower case by just setting a single bit (or lower case to upper case by clearing a bit).
All the numbers come together, in order from 0 to 9. Notice you can convert from a character to a number by subtracting '0'.
The first 32 codes are an unfortunate combination of codes describing messages (STX, ETX, NL), codes used for formatting text (CR, BS), and codes used for controlling the hardware itself (BEL).
The puncuation, unfortunately, is pretty much just scattered in among those major blocks.

Extensions to ASCII

The IBM PC defined an eight bit "extended ASCII" that used another 128 characters for various graphics and line-drawing purposes.

ASCII has been adopted as the basis of the ISO 8859-x family of character encodings; in these, the remaining 128 characters are used for various language-dependant extensions such as German umlauted vowels and the like.

Last modified: Mon Oct 31 13:20:49 MST 2005

	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0	NUL	SOH	STX	ETX	EOT	ENQ	ACK	BEL	BS	HT	LF	VT	FF	CR	SO	SI
1	DLE	DC1	DC2	DC3	DC4	NAK	SYN	ETB	CAN	EM	SUB	ESC	FS	GS	RS	US
2	SP	!	"	#	$	%	&	'	(	)	*	+	,	-	.	/
3	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
4	@	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
5	P	Q	R	S	T	U	V	W	X	Y	Z	[	\	]	^	_
6	`	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o
7	p	q	r	s	t	u	v	w	x	y	z	{	\|	}	~	DEL

	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0	NUL	SOH	STX	ETX	EOT	ENQ	ACK	BEL	BS	HT	LF	VT	FF	CR	SO	SI
1	DLE	DC1	DC2	DC3	DC4	NAK	SYN	ETB	CAN	EM	SUB	ESC	FS	GS	RS	US
2	SP	!	"	#	$	%	&	'	(	)	*	+	,	-	.	/
3	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
4	@	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
5	P	Q	R	S	T	U	V	W	X	Y	Z	[	\	]	^	_
6	`	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o
7	p	q	r	s	t	u	v	w	x	y	z	{	\|	}	~	DEL

	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0	NUL	SOH	STX	ETX	EOT	ENQ	ACK	BEL	BS	HT	LF	VT	FF	CR	SO	SI
1	DLE	DC1	DC2	DC3	DC4	NAK	SYN	ETB	CAN	EM	SUB	ESC	FS	GS	RS	US
2	SP	!	"	#	$	%	&	'	(	)	*	+	,	-	.	/
3	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
4	@	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
5	P	Q	R	S	T	U	V	W	X	Y	Z	[	\	]	^	_
6	`	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o
7	p	q	r	s	t	u	v	w	x	y	z	{	\|	}	~	DEL