A BCD adder adds decimal digits stored as four-bit groups. An ordinary binary sum may not be a valid BCD digit. When a digit sum exceeds nine, or produces a carry from the four-bit addition, add binary 0110 to correct it and form the decimal carry.
Worked example
Add BCD digits 8 and 7: 1000 + 0111 = 1111, which represents 15 in binary but is not a valid BCD digit. Add 0110 to obtain 1 0101. The carry represents one ten and 0101 represents five, giving BCD 0001 0101.
What to watch for
Apply correction independently to each decimal position, including its incoming decimal carry. Ordinary binary addition of the entire BCD word is insufficient.
Try it yourself
What BCD result represents 6 + 5?
Answer: The sum is decimal 11. After correction, the tens digit is 0001 and the units digit is 0001.
How to study this topic
State the operand width and signedness first. Work out a decimal answer, then compare it with the binary result. Keep result bits, carry, borrow and signed overflow distinct when interpreting the circuit's output.
Explore related programmes
These recorded programmes are upcoming. This article is general educational content, not a syllabus or course sample.
Digital Electronics