If you think back to one of the problem sets, you'll remember that the number of terms in this expression increases exponentially with the number of bits in the adder. This directive allows you to specify how many bits of carry to compute in 2-level logic. Lower n's use less chip area, but make the processor run a little slower. We'll see why in a minute. The extra bit on the bottom is for the purpose of creating the carry-in of 1 for subtraction.
B contains the complement of each of B's bits, in order.
Doing so can save you much frustration. If you cannot find a solution to your problem here, please check out the Xilinx Student Edition FAQ . When Trying to Synthesize Abel files, you get abl2edif has failed. Please see the abl2edif.log for detailed information. This error usually implies that your file name and your module name don't match.