| 摘要 |
<p>Carry lookahead techniques are adapted for implementation in a programmable logic device. In one example of the invention, a carry result is computed for a block of function cells, each function cell representing one bit in a multibit operation that uses carry. This carry result is combined with the carry input from a function cell block representing less significant bits in the operation and a carry output is provided to a function cell block representing more significant bits in the operation. The received carry can also be supplied to adjust provisional carry results for each bit associated with the function cells in the block. Accordingly, the received carry input need not be rippled through all the function cells in the block, thus reducing carry propagation delays. This technique is suitable for use in programmable logic devices because only minimal additional logic need be included in each block of function cells (such as the CLBs and LABs in the prior art), and because few, if any, new interconnections between blocks need be introduced.</p> |
