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Unformatted text preview: igure 4.1 you can see that only the first 10 of these combinations are used to represent
decimal digits. The remaining 6 arrangements (1010, 1011, 1100, 1101, 1110 and 1111)
have decimal values from 10 to 15. These arrangements are not used in BCD coding.
That is, 1010 does not represent lO,o in BCD. Instead,
In the above discussion, we have used a group of 4 bits to represent a digit (character) in
BCD. 4bit BCD coding system can be used to represent only decimal numbers because 4
bits are insufficient to represent the various characters used by a computer. Instead of
using 4 bits with only 16 possible characters, computer designers commonly use 6 bits to
represent characters in BCD code. In the 6bit BCD code, the four BCD numeric place
positions are retained, but two additional zone positions are added. With 6 bits, it is
possible to represent 64 (2 6) different characters. This is a sufficient number to code the
decimal digits (10), alphabetic letters (26), and other special characters (28). Figure 4.2
illustrates the coding of alphabetic and numeric characters in BCD.
In Chapter 3 we have seen the use of octal and hexadecimal number systems as shortcut
notations for binary. Because BCD is a 6bit code, it can be easily divided into two 3bit
groups. Each of these 3bit groups can be represented by 1 octal digit. Thus octal number
system is used as shortcut notation for memory dump by computers that use BCD code
for internal representation of characters. This results in a onetothree reductions in the
volume of memory dump. Figure 4.2 also shows the octal equivalent of the alphabetic
and numerical characters coded in BCD.48
I Foundations of Computing
Character
BCD Code
Octal Equivalent
A
B
C
D
E
F
G
H
I Zone
11
11
11
11
11
11
11
11
11 Digit
0001
0010
0011
0100
0101
0110
0111
1000
1001 61
62
63
64
65
66
67
70
71 J
K
L
M
N 10
10
10
10
10 0001
0010
0011
0100
0101 41
42
43
44
45 O
P
Q
R 10
10
10
' 10 0110
0111
1000
1001 46
47
50
51 S
T
U
V
w
X
Y
z 01
01
01
01
01
01
01...
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 Spring '14

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