Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [113]
Our goal here is to develop a coded character set so that a sentence such as
I have 27 sisters.
can be represented by a series of codes, each of which is a certain number of bits. Some of the codes will represent letters, some will representation punctuation marks, and some will represent numbers. There should even be a code that represents the space between words. There are 18 characters in that sentence (including the spaces between the words). The consecutive character codes for such a sentence are often referred to as a text string.
That we need codes for numbers in a text string such as 27 might seem odd because we've been using bits to represent numbers for many chapters now. We may be tempted to assume that the codes for the 2 and 7 in this sentence are simply the binary numbers 10 and 111. But that's not necessarily the case. In the context of a sentence such as this, the characters 2 and 7 can be treated like any other character found in written English. They can have character codes that are completely unrelated to the actual values of the numbers.
Perhaps the most economical code for text is a 5-bit code that originated in an 1874 printing telegraph developed by Emile Baudot (pronounced bawdoh), an officer in the French Telegraph Service; his code was adopted by the Service in 1877. This code was later modified by Donald Murray and standardized in 1931 by the Comité Consultatif International Télégraphique et Téléphonique (CCITT), which is now known as the International Telecommunication Union (ITU). The code is formally known as the International Telegraph Alphabet No. 2, or ITA-2, and it's more popularly known in the United States as Baudot, although it's more correctly called the Murray code.
In the twentieth century, Baudot was often used in teletypewriters. A Baudot teletypewriter has a keyboard that looks something like a typewriter, except that it has only 30 keys and a spacebar. Teletypewriter keys are actually switches that cause a binary code to be generated and sent down the teletypewriter's output cable, one bit after the other. A teletypewriter also contains a printing mechanism. Codes coming through the teletypewriter's input cable trigger electromagnets that print characters on paper.
Because Baudot is a 5-bit code, there are only 32 codes. The hexadecimal values of these codes range from 00h through 1Fh. Here's how the 32 available codes correspond to the letters of the alphabet:
Hex Code
Baudot Letter
Hex Code
Baudot Letter
00
10
E
01
T
11
Z
02
Carriage Return
12
D
03
O
13
B
04
Space
14
S
05
H
15
Y
06
N
16
F
07
M
17
X
08
Line Feed
18
A
09
L
19
W
0A
R
1A
J
0B
G
1B
Figure Shift
0C
I
1C
U
0D
P
1D
Q
0E
C
1E
K
0F
V
1F
Letter Shift
Code 00h isn't assigned to anything. Of the remaining 31 codes, 26 are assigned to letters of the alphabet and the other five are indicated by italicized words or phrases in the table.
Code 04h is the Space code, which is used for the space separating words. Codes 02h and 08h are labeled Carriage Return and Line Feed. This terminology comes from the typewriter. When you're typing on a typewriter and reach the end of a line, you push a lever or button that does two things. First, it causes the carriage to be moved to the right so that the next line begins at the left side of the paper. That's a carriage return. Second, the typewriter rolls the carriage so that the next line is underneath the line you just finished. That's the linefeed. In Baudot, separate keyboard keys generate these two codes. A Baudot teletypewriter printer responds to these two codes when