1 Carry Lookahead Logic Carry Generate Gi = Ai Bi must generate carry when A = B = 1 Carry Propagate Pi = Ai xor Bi carry in will equal carry out here.

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Presentation transcript:

1 Carry Lookahead Logic Carry Generate Gi = Ai Bi must generate carry when A = B = 1 Carry Propagate Pi = Ai xor Bi carry in will equal carry out here Si = Ai xor Bi xor Ci = Pi xor Ci Ci+1 = Ai Bi + Ai Ci + Bi Ci = Ai Bi + Ci (Ai + Bi) = Ai Bi + Ci (Ai xor Bi) = Gi + Ci Pi Sum and Carry can be reexpressed in terms of generate/propagate: a b cout nothing happen 0 1 cin propagate cin 1 0 cin propagate cin generate

2 Carry Lookahead Logic Reexpress 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 in a two-level logic network Variables are the adder inputs and carry in to stage 0!

3 Carry Lookahead Implementation Adder with Propagate and Generate Outputs Increasingly complex logic

4 Ripple Carry Adder Circuits Critical delay: the propagation of carry from low to high order stages late arriving signal two gate delays to compute CO 4 stage adder final sum and carry

5 Carry Lookahead Logic Cascaded Carry Lookahead Carry lookahead logic generates individual carries sums computed much faster