Electronic – How to determine if a Carry Look Ahead Adder Overflows

addercarry-look-aheaddigital-logic

I am implementing a 32 bit CLA Adder like how a 16 bit adder is implemented in Wikipedia

Problem is how do I determine if the block overflows? I will need the carry into bit 32 (which is now in the last 4 bit CLA Adder) and XOR with carry out of bit 32(Check difference in MSB)?

UPDATE

Is my overflow logic correct

where Cin is the carry into the whole 16 bit block, P* is the Block Propagate, G* the block generate, and carry into bit 32 (typo, bit 16 MSB actually)

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I thought about seeing what my brain could regurgitate of long ago explnatins, as opposed to commin sense observations, BUT here's a page which does it all very well indeed.

They explain a "nice" 'trick'.

Two's compelment overflow detection can be achieved by XORing the carry-in and the carry-out bo=its of the leftmost full adder.

Their diagram. See above ref for detailed comment:

enter image description here

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Electronic – How to determine overflow from an adder/subtractor

From http://en.wikipedia.org/wiki/Two%27s_complement:

"The last two bits of the carry row (reading right-to-left) contain vital information: whether the calculation resulted in an arithmetic overflow, a number too large for the binary system to represent (in this case greater than 8 bits). An overflow condition exists when these last two bits are different from one another. As mentioned above, the sign of the number is encoded in the MSB of the result."

So your example, when done in 3 bits, has an overflow, because the carry into the highest bit is 1, the carry out of it is 0. (Also easy to see intuitively: you start with two positive numbers and end with a negative number). You tried to make things a bit confusing by writing the result in 4 bits, but you omitted the carry out of the 3th bit. When done in 4 bits there is no overflow, because the last two carry bits (into and out of the 4th bit) are both 0.

The correct way to note youre addition (in three bits) is:

 011
  011
+ 001 (2s)
=======
  100

Electronic – Carry look ahead adder/subtractor & Overflow, Negative Bits

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

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Electronic – How to determine overflow from an adder/subtractor

Electronic – Carry look ahead adder/subtractor & Overflow, Negative Bits

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