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## Multi-operand Addition

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**Multi-operand Addition**• Consider the Following Addition: SUM = a[0]; for (i=1; i<N; i++) { SUM = SUM + a[i]; } a[7] a[6] a[5] a[4] a[3] a[2] a[1] a[0] a[7]+a[6] a[5]+a[4] a[3]+a[2] a[1]+a[0] a[7]+a[6]+a[5]+a[4] a[3]+a[2]+a[1]+a[0] a[7]+a[6]+a[5]+a[4]+a[3]+a[2]+a[1]+a[0]**Multi-operand Addition**a[7] a[6] a[5] a[4] a[3] a[2] a[1] a[0] a[7]+a[6] a[5]+a[4] a[3]+a[2] a[1]+a[0] a[7]+a[6]+a[5]+a[4] a[3]+a[2]+a[1]+a[0] a[7]+a[6]+a[5]+a[4]+a[3]+a[2]+a[1]+a[0] • O(lg2N) – Lower Bound – Theoretical Lower Limit • This is “Binary Reduction” Operation • Theoretical Time to Add Two Values • O(n) – Carry Ripple Operation • O(lg2n) – CLG/CLA tree/Prefix/Carry Skip/Carry Select • O(1) – Avizienis/Takagi Signed Digit Arithmetic**Multiplication**• Multiplication Requires Multi-operand Addition • Dot Product Requires Multi-operand Addition • Defer Carry Assimilation • Represent Intermediate Sums Redundantly**Carry Save Adders**• FA Used in This Configuration is Also Known as a 3:2 Compressor**Dot Notation**3:2 Compressor 2:2 Compressor**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 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**• 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**• 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**• 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**• 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 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**• 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 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**• 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**• 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**Single Full Adder Used Here With Result Fanned Out**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 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