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Wallace Versus Dadda Trees. Reduce the Number of Operands at Earliest Opportunity. m Dots ... A Solitary 1 in a Column is Replaced by a +1 in That. Column and a 1 ...

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Wallace Tree

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wallace tree
Wallace Tree
  • Previous Example is 7 Input Wallace Tree
  • n-input Wallace Tree Reduces k-bit Inputs to Two (k + log2n - 1)-bit Outputs
  • CSA Reduces Number of Operands by Factor of 1.5
  • Smallest Height h(n) For an n-input Tree Can be Given by a Recurrence Relation
wallace tree2
Wallace Tree
  • h(n)=1+h(2n/3)
  • Ignoring Ceiling Operator Write as: h(n)=1+h(2n/3)
  • Can Get Lower Bound on Tree Height: h(n)log1.5(n/2)
  • Equality for n=2, 3 only
wallace tree height
Wallace Tree Height
  • Can Also Consider n(h) – Number of Inputs for a Tree of Height h
  • Recurrence is: n(h) = 3n(h-1)/2
  • Ignoring Floor Operator Can get Bounds
  • Lower Bound: n(h) > 2(3/2)h-1
  • Upper Bound: n(h) < 2(3/2)h
  • Exact Values for 0  h  20 in Table
wallace versus dadda trees
Wallace Versus Dadda Trees
  • Reduce the Number of Operands at Earliest Opportunity
  • m Dots Per Column – Apply m/3 Full Adders to Column
  • Tends to Minimize Overall Delay by Making CPA CPA as Short as Possible
  • Delay of Fast CPA is Generally Not Smoothly Increasing Function of Word Width
  • EXAMPLE: CLA Has Essentially Same Delay for Widths of 17-32 Bits
  • Dadda Tree Reduces Number of Operands to Next Lower Number USing the Fewest FAs and HAs as Possible
  • Justification is No Need to Reduce Number of Operands to Next Lower n(h) in Tree Since A Faster Tree Would Not Result
parallel counters
Parallel Counters
  • Single-bit Full Adder Referred to as (3:2) Counter (or Compressor)
    • Meaning is it “Counts” the Ones in 3 Input Bits
  • Can be Generalized to (n : log2(n+1) Counter
    • Has n Inputs
    • Produces a log2(n+1)-bit Binary Output Representing the Number of 1’s Among the n Inputs
  • Next Example Shows a (10:4) Counter
generalized parallel counters
Generalized Parallel Counters
  • Parallel Counter Reduces Number of Dots in a Column (same Radix Position)
  • Output Dots are Placed into Different Positions (one each)
  • Can Generalize This Notion
  • Generalized Parallel Counter Receives “Dot Patterns” as Input (not Necessarily in Same Bit Position)
  • Converts Them to Other Dot Patterns (not Necessarily one in Each Column)
  • If Output Dot Pattern Has Fewer Dots Than Input, the Counter is a Compressor and Can be Used for a Tree
generalized parallel counters11
Generalized Parallel Counters
  • Characterized by Number of Dots in Each Input Column and Output Column
  • Book Limits to Class of Counters that Output a Single Dot in Each Column
  • Limitation Allows Output to be Characterized by Single Integer Representing Number of Columns Spanned by Output
  • Input Side is Characterized by Integer Sequence Corresponding to Number of Inputs in Various Columns
5 5 4 parallel counter
(5,5 : 4) Parallel Counter
  • Dot Notation for (5,5 : 4) Counter
  • (5,5 : 4) Counters to Compress 5 Numbers to 2 Numbers
  • Can Have Other Forms, eg. ( 4,6 : 4) Counter
    • Receives 6 bits of weight 1 and 4 bits of weight 2
    • Delivers the Weighted Sum in the Form of a 4-bit Binary Number
  • This Type Requires Sum of Output Weights to Equal or Exceed Sum of Input Weights
generalized parallel counters13
Generalized Parallel Counters
  • Powerful Concept – 4-bit Binary Full Adder Can be Viewed as (2,2,2,3 : 5)-counter
  • Goal is to Reduce n Numbers to 2 Numbers in Carry-Save Adder
  • Sometimes Notation of (n : 2)-counter is Used Although it Strictly Doesn’t Make Sense for n > 3
  • (n : 2)-counter is Shorthand Notation for a Slice of a Circuit
  • When Slice is Replicated, n Values are Reduced to 2 Values
  • Slice i Receives n Input Bits in Position i Plus Transfer (or Carry) Bits From One or More Positions to Right (i-1, i-2, etc.)
  • Slice i Produces Output Bits in Positions i and i+1 Plus Transfer Digits Into Higher Positions (i+1, i+2, etc.)
  • yj Denotes Number of Transfer bits From Slice i to i+j
n 2 parallel counters
(n : 2) Parallel Counters
  • Must Satisfy This Inequality for Scheme to Work
  • 3 Represents Maximum of 2 Output Bits
  • eg. (7 : 2)-counter can be Built Allowing y1=1 - Transfer bit From Position i to i+1 and y2=2 - Transfer bit into Position i+2
adding multiple signed numbers
Adding Multiple Signed Numbers
  • Must Sign Extend 2’s Complement Numbers to Final Result Width
  • Appears Sign Extension Could Dramatically Increase Complexity of CSA Tree for Large n
  • Trick is to Take Advantage of Fact that all Sign Extension bits are Identical
  • Use a Single Full Adder to do Job of Several Full Adders
  • Allows CSA Internal Widths to be Marginally Increased
hardware sharing method
Hardware Sharing Method

Single Full Adder Used Here With Result Fanned Out

negative weight interpretation
Negative Weight Interpretation
  • Recall That 2’s Complement Values May be Interpreted as:
  • Replace Negative Sign Bit by it’s Complement and Put a -1 in Sign Column
  • Multiple –1’s Can be Combined Each Pair Placed in –1 in Next Higher Column
  • A Solitary –1 in a Column is Replaced by a +1 in That Column and a –1 in the Next Higher Column
negative weight interpretation18
Negative Weight Interpretation
  • Complement Three Sign Bits and Place –1’s in Sign Column
  • Replace Three –1’s by a +1 in Sign Position and Two –1’s in Next Higher Position
  • These Two –1’s are Removed and Single –1 is Inserted in Position k+1
  • Latter –1 is in Turn Replaced by a +1 in Position k+1 and a –1 in Position k+2
  • Finally a –1 Moves Out of the Resultant Sum Width and the Procedure Stops