Carry-lookahead logic (cont'd)
Carries can be computed from gi and pi:
- C1 = g0 + p0 C0
- C2 = g1 + p1 C1 = g1 + p1 g0 + p1 p0 C0
- C3 = g2 + p2 C2 = g2 + p2 g1 + p2 p1 g0 + p2 p1 p0 C0
- C4 = g3 + p3 C3 = g3 + p3 g2 + p3 p2 g1 + p3 p2 p1 g0 + p3 p2 p1 p0 C0
The carry equations use two-level logic
- no intermediate carries are used, only the inputs
- this allows computation of all sum outputs to proceed in parallel
Can’t go past about 4 bits
- big gates are too slow
- circuit size grows quadratically