Signed number addition of two negative numbers and overflow condition.

EX1:  -7,  -6 : Two negative numbers.

Step 1: Convert to 2's complement for negative numbers.

-7 (in Binary 4’b1111) = 1 001, save msb as sign bit and convert lower 3 bits into 2’s complement.

-6 (in Binary 4’b1110) = 1 010, save msb as sign bit and convert lower 3 bits in 2’s complement.

Step 2: Add & store the result in sum register 5 bits wide.  

 1 001

 1 010 

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10 011

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Step 3: No carry into signed bit but carry out of signed bit. In this scenario keep MSB as sign and take 2’s complement of lower 4 bits.

Result: 5’b1 1101 = 0x1D = -13