Instead in the above discussion we have used a group

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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. 4-bit 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 6-bit 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 6-bit code, it can be easily divided into two 3-bit groups. Each of these 3-bit 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 one-to-three 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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