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Consider this GATE 2003 question: https://gateoverflow.in/937/gate2003-46

Here, instead of XOR gates we had OR gates, then which of the following operations can we perform?

$A + B, A - B\ and\ A + 1$

And what about A + 1. when K = 1 and C0 = 0 ??

it will not perform A+1 in this case, instead it will perform A-1....

it will be like,  an-1an-2an-3.....a0 + 1111.....11  // all 1's in 2's complement represent -1, so

finally it will be  an-1an-2an-3.....a0 - 1 = A-1

Thanks :)
@joshi_nitish The A-1 part was really good. Thanks.