Carry look ahead adder/subtractor & Overflow, Negative Bits

Question

I am suppose to design a carry look ahead adder with ...

Input flag bits

• E (Enable)
• S (Subtraction)

Output

• The usual sum & carry out
• O (Overflow)
• N (Negative)
• Z (Zero)

But the question is vague ... It says if

• E = 0, S = 1, do subtraction
• E = 0, S = 0, do addition
• then if E = 1? What do I do? What do usual circuits do? I suppose if 0 is enable, 1 is disable, then I put all outputs 0?

• For overflow bit, I suppose the below, where \$C_{in}, C_{out}\$ are carry in/out of MSB/Sign Bit and \$S\$ is subtract bit. The 1st half checks if Cin is different from Cout (of MSB), when doing subtraction. And the 2nd half checks if theres a carry out when doing addition. Is my implementation correct?

  

• For negative bit I also check if doing subtraction using \$S \cdot MSB\$?

Answer 1

Ask your professor if the question is vague. But your assumption that E=1 means disable is probably correct.