Wallace tree

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Wallace tree is an efficient hardware implementation of multiplication of two integers.

The Wallace tree has 3 steps:

1. Multiply (that is - AND) each bit of one of the arguments, by each bit of the other, yielding n² results. Depending on position of the multiplied bits, the wires carry different weights, for example wire of bit carrying result of a₂b₃ is 32.
2. Reduce the number of partial products to two by layers of full and half adders.
3. Group the wires in two numbers, and add them with a conventional adder.

The second phase works as follows. As long as there are 3 or more wires with the same weight add a following layer:

• Take any 3 wires with the same weights and input them into a full adder. The result will be an output wire of the same weight and an output wire with a higher weight for each 3 input wires.
• If there are 2 wires of the same weight left, input them into a half adder.
• If there is just 1 wire left, connect it to the next layer.

The benefit of the Wallace tree is that there are only O(logn) reduction layers, and each layer has O(1) propagational delay. As making the partial products is O(1) and the final addition is O(logn), the multiplication is only O(logn), not much slower than addition (however, much more expensive in the gate count). Naively adding partial products with regular adders would require O(logn)² time.

These computations only consider gate delays and don't deal with wire delays, which can also be very substantial.

The Wallace tree can be also represented by a tree of 3/2 or 4/2 adders.

It is sometimes combined with Booth encoding.

[edit] Weights Explained

2 to the power of the index, so
a₀b₀ - both have index of 0; 2⁰ * 2⁰ = 1, so the weight is 1.
a₃b₂ - have indexes of 3 and 2; 2³ * 2² = 32, so the weight is 32.

[edit] Example

n = 4, multiplying a₃a₂a₁a₀ by b₃b₂b₁b₀:

1. First we multiply every bit by every bit:
   • weight 1 - a₀b₀
   • weight 2 - a₀b₁, a₁b₀
   • weight 4 - a₀b₂, a₁b₁, a₂b₀
   • weight 8 - a₀b₃, a₁b₂, a₂b₁, a₃b₀
   • weight 16 - a₁b₃, a₂b₂, a₃b₁
   • weight 32 - a₂b₃, a₃b₂
   • weight 64 - a₃b₃
2. Reduction layer 1:
   • Pass the only weight-1 wire through, output: 1 weight-1 wire
   • Add a half adder for weight 2, outputs: 1 weight-2 wire, 1 weight-4 wire
   • Add a full adder for weight 4, outputs: 1 weight-4 wire, 1 weight-8 wire
   • Add a full adder for weight 8, and pass the remaining wire through, outputs: 2 weight-8 wires, 1 weight-16 wire
   • Add a full adder for weight 16, outputs: 1 weight-16 wire, 1 weight-32 wire
   • Add a half adder for weight 32, outputs: 1 weight-32 wire, 1 weight-64 wire
   • Pass the only weight-64 wire through, output: 1 weight-64 wire
3. Wires at the output of reduction layer 1:
   • weight 1 - 1
   • weight 2 - 1
   • weight 4 - 2
   • weight 8 - 3
   • weight 16 - 2
   • weight 32 - 2
   • weight 64 - 2
4. Reduction layer 2:
   • Add a full adder for weight 8, and half adders for weights 4, 16, 32, 64
5. Outputs:
   • weight 1 - 1
   • weight 2 - 1
   • weight 4 - 1
   • weight 8 - 2
   • weight 16 - 2
   • weight 32 - 2
   • weight 64 - 2
   • weight 128 - 1
6. Group the wires into a pair integers and an adder to add them.

See also: Dadda tree