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National Taiwan University


        8.2.7.3 Wallace Tree Multiplication
                              •   In effect, a “one’
                                                   s
                                  counter”: A, B, and
                                  C inputs and
                                  encodes them on
                                  SUM and CARRY
                                  outputs.

                              •   A 1-bit full adder
                                  (FA) provides a 3:2
                                  compression in the
                                  number of bits.

 A. Y. Wu                                        pp. 1
National Taiwan University


            Ex: 6×6 Multiplier(table)




 A. Y. Wu                               pp. 2
National Taiwan University


  Seven bits Wallace tree addition




 A. Y. Wu                       pp. 3
National Taiwan University


            Ex: 6×6 Wallace Multiplier




 A. Y. Wu                                pp. 4
National Taiwan University


                       Example
 •   In a 32-bit multiplier, the maximum number of partial
     products is 32 and the compressions are:

     32 → 22 → 16 → 12 → 8 → 6 → 4 → 3 → 2

     => There are 9 adder delays in the array


 1. c.f. Array multiplier (Booth-recoded) =16
 2. Can be used together with Booth-encoding scheme



 A. Y. Wu                                               pp. 5
National Taiwan University


            Wallace Tree Multiplier
   •    MxN Booth-encoded multiplier (IEEE JSSC,
        vol.1,no.2, June 1993)




 A. Y. Wu                                          pp. 6
National Taiwan University


  A typical delay distribution of the
   output of Wallace tree section




 A. Y. Wu                          pp. 7
National Taiwan University


                      Extension
  •    32-bits Wallace-tree multiplier has 9 adder delays
       in the array
  •    Questions:
       1. For a 64-bit multiplier, what is the minimum
          adder delay?
       2. Is there any way to reduce the delay by other
          compression scheme?




 A. Y. Wu                                                 pp. 8
National Taiwan University

                    Approach:
            Using 4:2 compression adder

                             •   The 4:2 compression
                                 (really 5:3) has three
                                 XOR delays in the
                                 sum path.
                             •   c.f.: Four XOR delays
                                 will be present if two
                                 adders are used



 A. Y. Wu                                          pp. 9
National Taiwan University


                      Reference
 •    A 54x54 regularly structured tree multiplier ,IEEE
      Journal of Solid-State Circuits (vol27 no.9, 1992)




 A. Y. Wu                                                  pp. 10

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Wallace tree multiplier

  • 1. National Taiwan University 8.2.7.3 Wallace Tree Multiplication • In effect, a “one’ s counter”: A, B, and C inputs and encodes them on SUM and CARRY outputs. • A 1-bit full adder (FA) provides a 3:2 compression in the number of bits. A. Y. Wu pp. 1
  • 2. National Taiwan University Ex: 6×6 Multiplier(table) A. Y. Wu pp. 2
  • 3. National Taiwan University Seven bits Wallace tree addition A. Y. Wu pp. 3
  • 4. National Taiwan University Ex: 6×6 Wallace Multiplier A. Y. Wu pp. 4
  • 5. National Taiwan University Example • In a 32-bit multiplier, the maximum number of partial products is 32 and the compressions are: 32 → 22 → 16 → 12 → 8 → 6 → 4 → 3 → 2 => There are 9 adder delays in the array 1. c.f. Array multiplier (Booth-recoded) =16 2. Can be used together with Booth-encoding scheme A. Y. Wu pp. 5
  • 6. National Taiwan University Wallace Tree Multiplier • MxN Booth-encoded multiplier (IEEE JSSC, vol.1,no.2, June 1993) A. Y. Wu pp. 6
  • 7. National Taiwan University A typical delay distribution of the output of Wallace tree section A. Y. Wu pp. 7
  • 8. National Taiwan University Extension • 32-bits Wallace-tree multiplier has 9 adder delays in the array • Questions: 1. For a 64-bit multiplier, what is the minimum adder delay? 2. Is there any way to reduce the delay by other compression scheme? A. Y. Wu pp. 8
  • 9. National Taiwan University Approach: Using 4:2 compression adder • The 4:2 compression (really 5:3) has three XOR delays in the sum path. • c.f.: Four XOR delays will be present if two adders are used A. Y. Wu pp. 9
  • 10. National Taiwan University Reference • A 54x54 regularly structured tree multiplier ,IEEE Journal of Solid-State Circuits (vol27 no.9, 1992) A. Y. Wu pp. 10