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FIGURE 9.5 16-bit carry lookahead adder.<br />

If r = 4<br />

© 2002 by CRC Press LLC<br />

s15 a15 b15 s11 a11 b11 Modified<br />

Full<br />

Adder<br />

p15 g15 c15 s 7 a 7<br />

Modified<br />

Full<br />

Adder<br />

p11 g11 c11 s14 a14 b14 b 7<br />

s 3 a 3<br />

Modified<br />

Full<br />

Adder<br />

p7 g7 c7 s10 a10 b10 Modified<br />

Full<br />

Adder<br />

p14 g14 c14 b 3<br />

Modified<br />

Full<br />

Adder<br />

p3 g3 c3 s 6 a 6<br />

Modified<br />

Full<br />

Adder<br />

p10 g10 c10 b 6<br />

Modified<br />

Full<br />

Adder<br />

p2 g2 c2 2 2<br />

GATESCLA ≈ 12 -- n – 4 --<br />

(9.14)<br />

3 3<br />

The carry lookahead approach reduces the delay of adders from increasing linearly with the word size<br />

(as is the case for ripple carry adders) to increasing in proportion to the logarithm of the word size. As with<br />

ripple carry adders, the carry lookahead adder complexity grows linearly with the word size (for r = 4,<br />

this occurs at a 40% faster rate than the ripple carry adders).<br />

s 2 a 2<br />

Modified<br />

Full<br />

Adder<br />

p6 g6 c6 b 2<br />

s 1 a 1<br />

b 1<br />

Modified<br />

Full<br />

Adder<br />

p1 g1 Lookahead Logic<br />

s 5 a 5<br />

b 5<br />

Modified<br />

Full<br />

Adder<br />

p5 g5 Lookahead Logic<br />

s 9 a 9<br />

b 9<br />

Modified<br />

Full<br />

Adder<br />

p9 g9 Lookahead Logic<br />

s13 a13 b13 Modified<br />

Full<br />

Adder<br />

p13 g13 c13 Lookahead Logic<br />

c 9<br />

s12 a12 b12 p 12:15<br />

Modified<br />

Full<br />

Adder<br />

p12 g12 g 12:15<br />

s 8 a 8<br />

c 5<br />

Modified<br />

Full<br />

Adder<br />

p8 g8 p 8:11 g 8:11<br />

c 12<br />

b 8<br />

s 4 a 4<br />

p 4:7<br />

b 4<br />

c 1<br />

Modified<br />

Full<br />

Adder<br />

p4 g4 c 8<br />

s 0 a 0<br />

g 4:7<br />

Lookahead Logic<br />

Modified<br />

Full<br />

Adder<br />

p0 g0 p 0:3<br />

c 4<br />

b 0<br />

g 0:3<br />

p 0:15 g 0:15<br />

c 0

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