Consider the Adder-Subtractor circuit as shown in Figure 3.15 page 76

Question

Consider the Adder-Subtractor circuit as shown in Figure 3.15 page 76 of Block 1. Explain how this circuit will perform subtraction (A-B), if the value of A is 1011 and B is 0011. You must list all the bit values including Cin and Cout and overflow, if any

07 Aug 2023

Answer :

Word Count : 455

In the standard 4-bit adder–subtractor, subtraction $A-B$ is performed by adding $A$ to the two’s complement of $B$. Hardware-wise, a mode control $M=1$ selects subtraction: each $B_i$ is XORed with $M$ (so $B$ is bitwise inverted), and the least significant carry-in is forced to $C_{in,0}=M=1$, which adds the “+1” to complete two’s complement. For the given inputs $A=1011$ and $B=0011$: * $A = a_3a_2a_1a_0 = 1\;0\;1\;1$ * $B = b_3b_2b_1b_0 = 0\;0\;1\;1$ * With $M=1$: $B' = B \oplus 1111 = \overline{B} = 1100$ and $C_{in,0}=1$ Bit-by-bit full-adder operation (LSB → _______ _________ ____ _________ __________ __________.
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