
Carry-Select and Conditional-Sum Adders 85
the connections are regular. 8 The number of cells is
N-- n + (n - 1) + (n - 3) + (n - 7)... + (n - (n/2 - 1)) + 1
og2nl (o 1)
= ~ (n --(2 i- 1)) + 1 --(n + 1)(log2n ) -- 2 i + 1
i=0 i=0
= (n + 1)(log2n ) --(n -- 1) + 1
= (n) (log 2 n -- 1) + log 2 n + 2 2.64
As can be seen from the previous expression, the number of cells of this scheme is
about twice that of the basic scheme. If the number of cells is too high, it is possible
to use an intermediate scheme, which has an intermediate maximum fanout as
well as an intermediate number of cells (see Exercise 2.22). ...