Which of the following statements is INCORRECT about BCD adder?

Concept

A one-digit BCD adder operates on two 4-bit 8421-BCD digits and an incoming carry. It first forms an ordinary binary sum; if that intermediate sum exceeds 1001 or produces a carry, the correction 0110 is added. The corrected low four bits must therefore encode one decimal digit in BCD, while a separate carry is passed to the next decimal position.

Application

1. The two input digits contribute 4 + 4 bit lines, and the carry-in contributes 1 line. Hence the stage has 4 + 4 + 1 = 9 input lines.

2. The corrected decimal digit uses 4 output bits, and the decimal carry uses 1 output line. Hence the stage has 4 + 1 = 5 output lines.

3. Its purpose is to add two decimal digits already represented in BCD, so the corrected result digit must remain in the BCD code set 0000 through 1001.

4. The statement allowing the result digit to be outside BCD conflicts with that output invariant; this is the incorrect statement.

Cross-check

For example, 7 + 8 gives the intermediate binary sum 1111. Adding the BCD correction 0110 produces a decimal carry of 1 and the low bits 0101, which encode 5 in BCD; the two output digits therefore represent 15. This confirms both the 4-bit BCD result-plus-carry structure and the need for a BCD-coded output.

Result

The incorrect statement is: “Its output should be a decimal digit that need not be in BCD.”