Discussion
In BCD, the decimal number 25 is represented as
Answer: A
Correct answer: 0010 0101
In binary coded decimal each decimal digit is encoded separately in four bits. So 25 becomes 2 and 5 encoded individually: 2 is 0010 and 5 is 0101, giving 0010 0101. This is not the same as the pure binary value of 25, which is 11001 - the second option, and the usual trap. BCD deliberately sacrifices compactness for ease of conversion. Because only ten of the sixteen four-bit patterns are used, the six combinations from 1010 to 1111 are invalid in BCD, and arithmetic must correct for them: when a BCD addition produces a result above 9 or generates a carry, 6 is added to bring it back into range. The code is used wherever numbers are displayed or entered digit by digit - calculators, digital clocks, instrument panels - because converting each digit to a seven-segment display needs no division. The waste is about 20 per cent of the available codes.
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