2.2.1: Binary Coded Decimal (BCD)

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Paul Penfield, Jr.
Massachusetts Institute of Technology via MIT OpenCourseWare

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A common way to represent the digits 0 - 9 is by the ten four-bit patterns shown in Table 2.1. There are six bit patterns (for example 1010) that are not used, and the question is what to do with them. Here are a few ideas that come to mind.

First, the unused bit patterns might simply be ignored. If a decoder encounters one, perhaps as a result of an error in transmission or an error in encoding, it might return nothing, or might signal an output error. Second, the unused patterns might be mapped into legal values. For example, the unused patterns might all be converted to 9, under the theory that they represent 10, 11, 12, 13, 14, or 15, and the closest digit is 9. Or they might be decoded as 2, 3, 4, 5, 6, or 7, by setting the initial bit to 0, under the theory that the first bit might have gotten corrupted. Neither of these theories is particularly appealing, but in the design of a system using BCD, some such action must be provided.

Table 2.1: Binary Coded Decimal
Digit	Code
0	0000
1	0001
2	0010
3	0011
4	0100
5	0101
6	0110
7	0111
8	1000
9	1001