Question

Use the adder subtraction of Fig. 4.7 that has the following values of input Mode (M) and data inputs (A) and (B). In each case, determine the values of the outputs S₃ S₂ S₁ S₀ and C₄.

            M            A                 B

            0          0111                0110

            1          101                  1010

            1         0000                 0001

The adder-subtractor circuit of Fig. 4-7 has the following values for input mode M and data inputs A and B. In each case, determine the values of the outputs: S₃ S₂ S₁ S₀ and C₄.

	M	A	B
a.	0	0111	0110
b.	1	0101	1010
c.	1	1000	0001

Answer 1

a) Mode is 0. 4-bit addition operation is performed. So, A(0111) + B(0110) gives 1101. where S3 = 1, S2 = 1, S1 = 0, S0 = 1 and C4 = 0 because there is no carry.

b) Mode is given as 1. So, 4-bit subtraction operation is performed on A and B.

A(0101) - B(1010) = 1) 1011 . where S3 = 1, S2 = 0, S1 = 1, S0 = 1 and C4 = 1 because there is 1 bit borrow. Result in decimal form is -5.

c) Mode is given as 1. So, 4-bit subtraction operation is performed on A and B.

A(1000) - B(0001) = 0111 . where S3 = 0, S2 = 1, S1 = 1, S0 = 1 and C4 = 0 because there is no borrow.