PPT Slide
4 – Combinational Logic Design Examples
Carry-lookahead logic (cont'd)
Re-express the carry logic as follows:
- 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
Each of the carry equations can be implemented with two-level logic
- all inputs are now directly derived from data inputs and not from intermediate carries
- this allows computation of all sum outputs to proceed in parallel