Assignment No. 3
Assignment No. 3
BCD Adder:
BCD adder is a circuit that adds two BCD digits & produces a sum of digits also in BCD.
Rules for BCD addition:
1. Add two numbers using rules of Binary addition.
2. If the 4 bit sum is greater than 9 or if carry is generated then the sum is invalid. To correct the sum add 0110 i.e. (6)10 to sum. If carry is generated from this addition add it to next higher order BCD digit.
3. If the 4 bit sum is less than 9 or equal to 9 then sum is in proper form.
CASE I: Sum <= 9 & carry = 0
Add BCD digits 3 & 4
1. 0 0 1 1
+ 0 1 0 0
---------
0 1 1 1
Answer is valid BCD number = (7) BCD & so 0110 is not added.
CASE II: Sum > 9 & carry = 0
Add BCD digits 6 & 5
1. 0 1 1 0
+ 0 1 0 1
-----------
10 1 1
Invalid BCD (since sum > 9) so 0110 is to be added
2. 1 0 1 1
+ 0 1 1 0
-----------
1 0 0 0 1
(1 1)BCD
Valid BCD result = (11) BCD
CASE III: Sum < = 9 & carry = 1
Add BCD digits 9 & 9
1. 1 0 0 1
+ 1 0 0 1
-----------
1 0 0 1 0
Invalid BCD (since Carry = 1 ) so 0110 is to be added
2. 1 0 0 1 0
+ 0 1 1 0
------------
1 1 0 0 0
(1 8)BCD
Valid BCD result = (18) BCD