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Conditional-Sum Adders and Parallel Prefix Network Adders

ECE 645: Lecture 3. Conditional-Sum Adders and Parallel Prefix Network Adders . Required Reading. Behrooz Parhami, Computer Arithmetic: Algorithms and Hardware Design. Chapter 7 .4, Conditional-Sum Adder Chapter 6 .4, Carry Determination as Prefix Computation

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Conditional-Sum Adders and Parallel Prefix Network Adders

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  1. ECE 645: Lecture 3 Conditional-Sum Adders and Parallel Prefix Network Adders

  2. Required Reading Behrooz Parhami, Computer Arithmetic: Algorithms and Hardware Design Chapter 7.4, Conditional-Sum Adder Chapter 6.4, Carry Determination as Prefix Computation Chapter 6.5, Alternative Parallel Prefix Networks

  3. Conditional-Sum Adders

  4. One-level k-bit Carry-Select Adder

  5. Two-level k-bit Carry Select Adder

  6. Conditional Sum Adder • Extension of carry-select adder • Carry select adder • One-level using k/2-bit adders • Two-level using k/4-bit adders • Three-level using k/8-bit adders • Etc. • Assuming k is a power of two, eventually have an extreme where there are log2k-levels using 1-bit adders • This is a conditional sum adder

  7. Conditional Sum Adder:Top-Level Block for One Bit Position

  8. Three Levels of a Conditional Sum Adder xi xi+3 yi+3 yi+2 yi+1 yi xi+2 xi+1 branch point 1-bit conditional sum block concatenation c=1 c=1 c=1 c=1 c=0 c=0 c=0 c=0 2 2 2 2 2 2 2 2 1 1 1+1 1 1 2 2 1 1 2 2 1 1 c=0 c=0 c=1 c=1 3 3 3 3 1 2+1 1 2 2 3 3 block carry-indetermines selection 5 4+1 5 c=0 c=1

  9. 16-Bit Conditional Sum Adder Example

  10. Conditional Sum Adder Metrics

  11. Parallel Prefix Network Adders

  12. Parallel Prefix Network Adders Basic component - Carry operator (1) g p B” B’ B g” g’ p” p’ g = g” + g’p” p = p’p” (g, p) = (g’, p’) ¢ (g”, p”) = (g” + g’p”, p’p”)

  13. Parallel Prefix Network Adders Basic component - Carry operator (2) g p overlap okay! B” B’ B g” g’ p” p’ g = g” + g’p” p = p’p” (g, p) = (g’, p’) ¢ (g”, p”) = (g” + g’p”, p’p”)

  14. Properties of the carry operator ¢ Associative [(g1, p1) ¢ (g2, p2)] ¢ (g3, p3) = (g1, p1) ¢ [(g2, p2) ¢ (g3, p3)] Not commutative (g1, p1) ¢ (g2, p2)  (g2, p2) ¢ (g1, p1)

  15. Parallel Prefix Network Adders Major concept Given: (g0, p0) (g1, p1) (g2, p2) …. (gk-1, pk-1) Find: (g[0,0], p[0,0]) (g[0,1], p[0,1]) (g[0,2], p[0,2]) … (g[0,k-1], p[0,k-1]) block generate from index 0 to k-1 ci = g[0,i-1] + c0p[0,i-1]

  16. Similar to Parallel Prefix Sum Problem Parallel Prefix Sum Problem Given: x0 x1 x2 … xk-1 Find: x0 x0+x1 x0+x1+x2 … x0+x1+x2+ …+ xk-1 Parallel Prefix Adder Problem Given: x0 x1 x2 … xk-1 Find: x0 x0 ¢ x1 x0 ¢ x1 ¢ x2 … x0 ¢ x1 ¢ x2 ¢ … ¢ xk-1 where xi = (gi, pi)

  17. Parallel Prefix Sums Network I

  18. Parallel Prefix Sums Network I – Cost (Area) Analysis Cost = C(k) = 2 C(k/2) + k/2 = = 2 [2C(k/4) + k/4] + k/2 = 4 C(k/4) + k/2 + k/2 = = …. = = 2 log k-1C(2) + k/2 (log2k-1) = = k/2 log2k 2 C(2) = 1 Example: C(16) = 2 C(8) + 8 = 2[2 C(4) + 4] + 8 = = 4 C(4) + 16 = 4 [2 C(2) + 2] + 16 = = 8 C(2) + 24 = 8 + 24 = 32 = (16/2) log2 16

  19. Parallel Prefix Sums Network I – Delay Analysis Delay = D(k) = D(k/2) + 1 = = [D(k/4) + 1] + 1 = D(k/4) + 1 + 1 = = …. = = log2k D(2) = 1 Example: D(16) = D(8) + 1 = [D(4) + 1] + 1 = = D(4) + 2 = [D(2) + 1] + 2 = = 4 = log2 16

  20. Parallel Prefix Sums Network II (Brent-Kung)

  21. Parallel Prefix Sums Network II – Cost (Area) Analysis Cost = C(k) = C(k/2) + k-1 = = [C(k/4) + k/2-1] + k-1 = C(k/4) + 3k/2 - 2 = = …. = = C(2) + (2k - 2k/2(log k-1)) - (log2k-1) = = 2k - 2 - log2k 2 C(2) = 1 Example: C(16) = C(8) + 16-1 = [C(4) + 8-1] + 16-1 = = C(2) + 4-1 + 24-2 = 1 + 28 - 3 = 26 = 2·16 - 2 - log216

  22. Parallel Prefix Sums Network II – Delay Analysis Delay = D(k) = D(k/2) + 2 = = [D(k/4) + 2] + 2 = D(k/4) + 2 + 2 = = …. = = 2 log2k - 1 D(2) = 1 Example: D(16) = D(8) + 2 = [D(4) + 2] + 2 = = D(4) + 4 = [D(2) + 2] + 4 = = 7 = 2 log2 16 - 1

  23. 8-bit Brent-Kung Parallel Prefix Network

  24. 4-bit Brent-Kung Parallel Prefix Network x7’ x5’ x3’ x1’ 2 –bit B-K PPN s3’ s1’ s7’ s5’

  25. 8-bit Brent-Kung Parallel Prefix Network Adder

  26. GP c C S Critical Path gi = xi yi pi = xi yi 1 gate delay g= g” + g’ p” p = p’ p” 2 gate delays ci+1 = g[0,i] + c0 p[0,i] 2 gate delays si = pi ci 1 gate delay

  27. Brent-Kung Parallel Prefix Graph for 16 Inputs

  28. Kogge-Stone Parallel Prefix Graph for 16 Inputs

  29. Parallel Prefix Network Adders Comparison of architectures Hybrid Network 2 Brent-Kung Kogge-Stone Delay(k) 2 log2k - 2 log2k log2k+1 Cost(k) 2k - 2 - log2k k/2 log2k k log2k - k + 1 6 5 Delay(16) 4 32 49 Cost(16) 26 Delay(32) 8 6 5 80 129 57 Cost(32)

  30. Latency vs. Area Tradeoff

  31. Hybrid Brent-Kung/Kogge-Stone Parallel Prefix Graph for 16 Inputs

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