1. Multioperand Addition
Lecture 4
Multioperand Addition

2. Required Reading Behrooz Parhami,
Computer Arithmetic: Algorithms and Hardware Design
Chapter 8, Multioperand Addition

3. Applications of multioperand addition
Inner product
Multiplication
n-1
n-1
p=a·x
s =  x(i) y(i) = 
p(i)
i=0
i=0

4. Number of bits of the result
S =  x(i)
x(i) [0..2k-1]
i=0
Smin = 0
Smax = n (2k-1)
# of bits of S = log2 (Smax + 1) =
= log2 (n (2k-1) + 1)  log2 n 2k =
= k + log2 n

5. Serial implementation of multioperand addition

6. Adding 7 numbers in the binary tree of adders

7. Ripple-carry adders at levels i and i+1

8. Example: Adding 8 3-bit numbers

9. Ripple-Carry Carry Propagate Adder (CPA)
an-1
bn-1 a2 b2 a1 b1 a0 b0 cn c3
cn-1 c2 c1 c0 FA
. . . FA FA FA
sn-1 s2 s1 s0

10. Carry Save Adder (CSA) an-1 bn-1 cn-1 a2 b2 c2 a1 b1 c1 a0 b0 c0 FA
. . . FA FA FA cn
sn-1
cn-1 s3 c3 s2 c2 s1 c1 s0

11. A Ripple-Carry vs. Carry-Save Adder

12. Operation of a Carry Save Adder (CSA)
Example 24 23 22 21 20 x y z s c
x+y+z = s + c

13. Carry propagate and carry-save adders
in dot notation

14. Specifying full- and half-adder blocks
in dot notation

15. Carry-save adder for four operands
x3 x2 x1 x0
y3 y2 y1 y0
z3 z2 z1 z0
w3 w2 w1 w0
s3 s2 s1 s0
c4 c3 c2 c1
c4 s3 s2 s1 s0 ’
S5 S4 S3 S2 S1 S0

16. Carry-save adder for four operands’ c4 ’ s3 ’ c3 ’ s2 ’ c2 ’ s1 ’ c1 ’

17. Carry-save adder for four operandsx y z w 4 4 4 4
CSA c s
CSA c’ s’
CPA S

18. Carry-save adder for six operands
Implementation of
one-bit slice
CSA tree

19. Tree of carry save adders reducing
seven numbers to two

20. Addition of seven
six-bit numbers
in dot notation

21. Adding seven
k-bit numbers:
block diagram

22. Number of Inputs and Tree Height
Relationship Between
Number of Inputs and Tree Height

23. Parameters of tree carry-save adders (1)
Latency
LatencyCSA = h(n)  TFA + LatencyCPA(k, n)
Tree height
for n operands
Component Adders
Widths
typically
close to k bits
CSA
k .. k + log2 n
CPA
 k + log2 n

24. Parameters of tree carry-save adders (2)
Maximum number of inputs that can be reduced
to two by an h-level tree, n(h)
n(0) = 2 3 2
n(h) = n(h-1)
n(1) = 3
n(2) = 4
n(3) = 6
n(4) = 9
n(5) = 13
n(6) = 19 3 2 3 2
2 ( )h-1 < n(h)  2 ( )h

25. Smallest height of the tree carry save adder
Parameters of tree carry-save adders (3)
Smallest height of the tree carry save adder
for n operands, h(n)
h(n) = 1 + h( ) 2 n 3
h(2) = 0
h(n)  log ( ) n 3 2 2

27. Wallace vs. Dadda Trees (1)
Wallace trees
Reduce the size of the final Carry Propagate Adder (CPA)
Optimum from the point of view of speed
Dadda trees
Reduce the cost of the carry save tree
Optimum (among the CSA trees) from the point of
view of area

28. Wallace vs. Dadda Trees (2)
Wallace reduces number of operands at earliest opportunity
Goal of this is to have smallest number of bits for CPA adder
However, sometimes having a few bits longer CPA adder does not affect the propagation delay significantly (i.e. carry-lookahead)
Dadda seeks to reduce the number of FA and HA units
May be at the cost of a slightly larger final CPA

29. 5-to-3 Parallel Counter a+b+c+d+e = s0+s1+s2 a 0 1 0 1 0 b 1 1 0 1 1 c24 23 22 21 20 a b c d e s0 s1 s2
a+b+c+d+e = s0+s1+s2

30. Implementation of 1-bit of 5-to-3 parallel counter
using single CLB slice of a Virtex FPGA

31. Carry Save Adder vs. 5-to-3 Parallel Counterb c d e w PC
CSA
CPA
y=a+b+c+d+e mod 2w a b c d e s2 s1 s0 w w w w w
CSA
CSA
CSA
CPA w
y=a+b+c+d+e mod 2w

32. Generalized Parallel Counters
Fig Dot notation for a (5, 5; 4)-counter and the use of such counters for reducing five numbers to two numbers.
Multicolumn reduction
(5, 5; 4)-counter
Unequal columns
Generalized parallel counter =
Parallel compressor
(2, 3; 3)-counter

34. 8.6 Modular Multioperand Adders
(a) m = 2k
Drop
(b) m = 2k – 1
(c) m = 2k + 1
Invert
Fig Modular carry-save addition with special moduli.
Apr. 2010
Computer Arithmetic, Addition/Subtraction