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So the ripple carry is the result of synthesized hardware, as interpreted by synthesizer. Its a parallel 256 sized combinational logic, which got translated to ripple carry adder, and the critical path includes most of the adders, and hence lead to a negative slack of -0.265 nsec.
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So are you saying this is a 256 bit adder? 256 bit op1 plus 256 bit op2 gives a 256b result? Your answer above is unclear.
If indeed it is a 256 bit adder I'm not surprised at all a ripple carry implementation does not meet timing. You need to implement a multi level carry lookahead design (over four or eight bit operand slices) to improve performance.
If it is not a 256 bit wide adder then I don't understand your response.
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