Programmable logic block with carry chains providing lookahead functions of different lengths

A programmable logic block provides N-bit and M-bit (e.g., (N/2)-bit) lookahead functionality for carry chains traversing the logic block, N and M being integers greater than one. An exemplary programmable logic block includes four carry multiplexers that together form a 4-bit lookahead carry chain. The 4-bit lookahead carry chain also provides a 2-bit lookahead output after the second carry multiplexer. Alternatively, the last two bits of the 4-bit lookahead carry chain can be used as a 2-bit lookahead carry chain. In one embodiment, the programmable logic block also includes four function generators associated with the four carry multiplexers. Each function generator drives a select terminal of the associated carry multiplexer. The 4-bit and 2-bit carry chains can be programmably coupled to an interconnect structure of the PLD at the carry out output terminals. In some embodiments, an initialization value can also be provided to the 4-bit and 2-bit carry chains.