Ones' complement adder/subtractor

dwight elvey dkelvey at
Sat Oct 2 22:45:35 CDT 2010

> From: eric at
> Dwight wrote:
> > Another possible is to put an incrementor on the output.
> > This is a simpler circuit than a full adder.
> I wrote:
> >> In "discrete" logic, or in an ASIC, it is simpler. In most FPGAs, it 
> >> will end up taking the same resources and having the same performance 
> >> as a full-adder.
> Dwight wrote:
> > Half area but just as long in time.
> Same area in FPGA, not half. Almost all FPGAs use LUTs with four or 
> more inputs, and half-adders generally end up taking a full LUT, just as 
> a full-adder does. The exception would be when the incrementer 
> (half-adders) could be merged with the previous stage of logic, but when 
> the incrementer follows a full-adder, you can't merge them, because they 
> each need a separate carry chain.
> Eric

Haaa! Evil FPGA!
Would it be possible to put two half adders in a single LUT?

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