An Approach for Fast BCD Addition

Binary coded decimal (BCD) can represent decimal numbers 0 to 9 in 4-bit binary equivalents. Figure 1 shows unpacked BCD representations of decimal digits. Unpacked BCD representation allows only one decimal digit per byte. This means that in an unpacked representation of any BCD number the upper four bits are zero. It is possible to pack two decimal digits into a single byte; this is known as packed BCD representation. The value of each 4-bit can range from 0 to 9 although a 4-bit data allows value from 0 to 15. The addition of unpacked or packed BCD numbers follows the general rule of binary addition. If during the addition process an invalid BCD number is produced or a carry is generated then an adjustment is necessary to correct the sum. This is traditionally done by adding 6 to the sum.

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