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

Input flag bits

  • E (Enable)
  • S (Subtraction)


  • 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?

    enter image description here

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


1 Answer 1


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

  • \$\begingroup\$ I read on and discovered when E = 1, do negation ... my mistake \$\endgroup\$
    – Jiew Meng
    Commented Sep 20, 2011 at 22:18

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