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Wallace Tree Adder


Design of Wallace Tree Adders :


There are many cases where it is desired to add more than two numbers together. The straightforward way of adding together m numbers (all n bits wide) is to add the first two, then add that sum to the next, and so on. This requires a total of m − 1 additions, for a total gate delay of O(m lg n) (assuming lookahead carry adders). Instead, a tree of adders can be formed, taking only O(lg m · lg n) gate delays.


A Wallace tree adder adds together n bits to produce a sum of log2n bits. The design of a Wallace tree adder to add seven bits (W7) is illustrated below:


An adder tree to add three 4-bit numbers is shown below:

An adder tree (interconnections incomplete) to add five 4-bit numbers is shown below:

The design of a 4-bit Wallace tree adder to add seven 4-bit numbers is illustrated below:


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