28
Digital Electronics
Traditional character encodings such as ASCII, EBCDIC and their variants have a limitation in
terms of the number of characters they can encode. In fact, no single encoding contains enough
characters so as to cover all the languages of the European Union. As a result, these encodings do
not permit multilingual computer processing. Unicode, developed jointly by the Unicode Consortium
and the International Standards Organization (ISO), is the most complete character encoding scheme
that allows text of all forms and languages to be encoded for use by computers. Different codes are
described in the following.
2.4.1 ASCII code
The ASCII (American Standard Code for Information Interchange), pronounced ‘ask-ee’, is strictly a
seven-bit code based on the English alphabet. ASCII codes are used to represent alphanumeric data
in computers, communications equipment and other related devices. The code was first published as
a standard in 1967. It was subsequently updated and published as ANSI X3.4-1968, then as ANSI
X3.4-1977 and finally as ANSI X3.4-1986. Since it is a seven-bit code, it can at the most represent
128 characters. It currently defines 95 printable characters including 26 upper-case letters (A to Z),
26 lower-case letters (a to z), 10 numerals (0 to 9) and 33 special characters including mathematical
symbols, punctuation marks and space character. In addition, it defines codes for 33 nonprinting, mostly
obsolete control characters that affect how text is processed. With the exception of ‘carriage return’
and/or ‘line feed’, all other characters have been rendered obsolete by modern mark-up languages and
communication protocols, the shift from text-based devices to graphical devices and the elimination of
teleprinters, punch cards and paper tapes. An eight-bit version of the ASCII code, known as US ASCII-8
or ASCII-8, has also been developed. The eight-bit version can represent a maximum of 256 characters.
Table 2.6 lists the ASCII codes for all 128 characters. When the ASCII code was introduced, many
computers dealt with eight-bit groups (or bytes) as the smallest unit of information. The eighth bit was
commonly used as a parity bit for error detection on communication lines and other device-specific
functions. Machines that did not use the parity bit typically set the eighth bit to ‘0’.
Table 2.6 ASCII code.
Decimal Hex
Binary
Code Code description
0
00
0000 0000 NUL
Null character
1
01
0000 0001 SOH
Start of header
2
02
0000 0010 STX
Start of text
3
03
0000 0011 ETX
End of text
4
04
0000 0100 EOT
End of transmission
5
05
0000 0101 ENQ
Enquiry
6
06
0000 0110 ACK Acknowledgement
7
07
0000 0111 BEL
Bell
8
08
0000 1000 BS
Backspace
9
09
0000 1001 HT
Horizontal tab
10
0A
0000 1010 LF
Line feed
11
0B
0000 1011 VT
Vertical tab
12
0C
0000 1100 FF
Form feed
13
0D
0000 1101 CR
Carriage return
14
0E
0000 1110 SO
Shift out
15
0F
0000 1111 SI
Shift in
16
10
0001 0000 DLE
Data link escape
17
11
0001 0001 DC1
Device control 1 (XON)
Digital Electronics
Traditional character encodings such as ASCII, EBCDIC and their variants have a limitation in
terms of the number of characters they can encode. In fact, no single encoding contains enough
characters so as to cover all the languages of the European Union. As a result, these encodings do
not permit multilingual computer processing. Unicode, developed jointly by the Unicode Consortium
and the International Standards Organization (ISO), is the most complete character encoding scheme
that allows text of all forms and languages to be encoded for use by computers. Different codes are
described in the following.
2.4.1 ASCII code
The ASCII (American Standard Code for Information Interchange), pronounced ‘ask-ee’, is strictly a
seven-bit code based on the English alphabet. ASCII codes are used to represent alphanumeric data
in computers, communications equipment and other related devices. The code was first published as
a standard in 1967. It was subsequently updated and published as ANSI X3.4-1968, then as ANSI
X3.4-1977 and finally as ANSI X3.4-1986. Since it is a seven-bit code, it can at the most represent
128 characters. It currently defines 95 printable characters including 26 upper-case letters (A to Z),
26 lower-case letters (a to z), 10 numerals (0 to 9) and 33 special characters including mathematical
symbols, punctuation marks and space character. In addition, it defines codes for 33 nonprinting, mostly
obsolete control characters that affect how text is processed. With the exception of ‘carriage return’
and/or ‘line feed’, all other characters have been rendered obsolete by modern mark-up languages and
communication protocols, the shift from text-based devices to graphical devices and the elimination of
teleprinters, punch cards and paper tapes. An eight-bit version of the ASCII code, known as US ASCII-8
or ASCII-8, has also been developed. The eight-bit version can represent a maximum of 256 characters.
Table 2.6 lists the ASCII codes for all 128 characters. When the ASCII code was introduced, many
computers dealt with eight-bit groups (or bytes) as the smallest unit of information. The eighth bit was
commonly used as a parity bit for error detection on communication lines and other device-specific
functions. Machines that did not use the parity bit typically set the eighth bit to ‘0’.
Table 2.6 ASCII code.
Decimal Hex
Binary
Code Code description
0
00
0000 0000 NUL
Null character
1
01
0000 0001 SOH
Start of header
2
02
0000 0010 STX
Start of text
3
03
0000 0011 ETX
End of text
4
04
0000 0100 EOT
End of transmission
5
05
0000 0101 ENQ
Enquiry
6
06
0000 0110 ACK Acknowledgement
7
07
0000 0111 BEL
Bell
8
08
0000 1000 BS
Backspace
9
09
0000 1001 HT
Horizontal tab
10
0A
0000 1010 LF
Line feed
11
0B
0000 1011 VT
Vertical tab
12
0C
0000 1100 FF
Form feed
13
0D
0000 1101 CR
Carriage return
14
0E
0000 1110 SO
Shift out
15
0F
0000 1111 SI
Shift in
16
10
0001 0000 DLE
Data link escape
17
11
0001 0001 DC1
Device control 1 (XON)