Adders


"FA" cell

FA cell

In the "FA" cell, the weighted sum of the output bits equals the weighted sum of the output bits, i.e. " x + y + z = 2 × c + s ". The three input bits share the same weight. Let it be "1". The output bit "s" has the same weight, while the output bit "c" weight is double (×2).
The "FA" cell conserves the sum just like the node conserves the electric current in the "Kirchoff's current law".
The "FA" cell is also called "3 Þ2 compressor" since it reduces the bits number from 3 to 2 while preserving the numerical value.

Carry propagate adder

The addition is by far the most common arithmetic operation in digital processors. Addition is itself very frequent and is also the basis of most other arithmetic operations like multiplication, division, square root extraction and elementary functions.
All "consistent" "FA" cells network preserves the property: the weighed sum of the output bits equals the weighted sum of the input bits.
To construct the adder S = A + B, the input bits come from the two numbers A and B and the output bits form the number S.
The number of "FA" cells is the number of bits of A and B.

Signed adder

The vertical arrow "view" next to the blue push-down button displays a double interpretation of the bit vectors A, B and S: unsigned integers (top) or signed integers in two's complement notation (bottom). The same adder supports both interpretations. One faces the following dilemma:

  • The bit sn is simply ignored. Then S has the same number of bits as the addends A and B but an overflow is possible. The overflow detection circuit is different for signed or unsigned addition.
  • The bit sn is part of the sum S. Then overflow is impossible but S has one more bit than A and B

The first solution is usually preferred. The vertical arrow that appears close to S toggles between the two possible S : either S truncated to n bits or S complete with n+1 bits.

Performance of the carry ripple adder

Let us assume that all the possible values for A and B are equiprobable and independent:

 

minimum

average

maximum

delay

0

log2(n)

n

activity

0

3n / 2

n2 / 2

The maximum delay (worst case) is usually not acceptable. Let us examine the carry propagation path that causes this delay.

Carry propagation path

For each "FA" cell, one of the three following cases happens :

  • the carry ci+1 is set to '0', noted 'K', if ai = 0 and bi = 0
  • the carry ci+1 is set to '1', noted 'G', if ai = 1 and bi = 1
  • the carry ci+1 is propagated, noted 'P', if (ai = 0 and bi = 1) or (ai = 1 and bi = 0). In this last case ci+1 = ci. This is the unfavorable case, materialized by an horizontal arrow in the next applet.
  The three case 'K', 'G' and 'P' are encoded onto two 2 bits. Check the box above to show the 2 bits. A "HA" cell selects the case according to ai and bi.

"BK" cell
(Brent & Kung)

The "BK" cell computes the carry for two binary positions ( two "FA" cells) or more generally two blocks of "FA" cells.

Sklansky's adder

To design fast adders, binary trees of "BK" cells will first generate simultaneously all the carries ci. The "Sklansky's adder" builds recursively 2-bit adders then 4-bit adders, 8-bit adders, 16-bit adder and so on by abutting each time two smaller adders. The architecture is simple and regular, but suffers from fan-out problems. Besides in some cases it is possible to use less "BK" cells with the same addition delay.

  The output bits si = ai Å bi Å ci. Now ai Å bi = '1' if the "HA" cell output equals 'P' . Thus the "HA" cell computes ai Å bi and subsequently si is given by one "XOR" gate.
The "BK" cells that output the carries ci never output the value 'P', consequently they can be simplified. Those "BK" cells are in yellow.

Associativity and non commutativity

The "BK" cell is associative as can be check on the applet at right, but its truth table shows clearly that is is not commutative ('G' and 'K' do not permute).

Fast adders
(Brent & Kung)

In a fast adder, all the carries ci are computed simultaneously through a binary tree of "BK" cells. To save on complexity, sharable intermediate results are computed once. There is only one rule to construct the trees: every tree output at position i must be connected to all inputs of position less than or equal to i by a planar binary tree of "BK" cells.
The rule simplicity usually allows many correct constructions.

 

In the "BK" cell tree, one may trade cells for delay, usually for the same addition the shorter the delay, the more "BK" cells are needed.

  • change the number of bits and/or the delay
  • check that the trees comply with the construction rule by clicking on a signal (a line), notice that each signal is named by a pair of integers .
  • simulate the carries computation by clicking on the keys .
  • display the tree construction process by clicking the key "Details"
  • display the adder VHDL description by clicking the key "VHDL". To save the VHDL description, select it, then copy and past it into a text editor.
  • display the table describing the trees by hitting the arrow "view".

Kogge & Stone adders

The binary trees of "BK" cells in the Kogge and Stone adders are not sharing. Consequently the signal fan-out is reduced to the minimum at the expense of more "BK" cells. Since the delay increases with the fan-out, it is now a bit shorter.

Ling adder

In the Ling's adder, the "BK" trees give a primitive called "pseudo carry". It avoids the computation of pi and gi, but on the other hand the carry has to be deducted from the "pseudo carry". The trick is that this late computation is overlapped by the "BK" cells delays. Consequently this adder is faster (a little bit) than the corresponding Kogge and Stone's adder. The VHDL synthesis from the applet takes advantage of Ling's approach if the box is checked.

"BK" cell trees editor

Check your understanding of "Brent-Kung" adders by creating new ones. Click on a position in the table to select it and then enter a number with the keyboard. Inconsistent values are replaced by 0 (no cell). The arrow "view" displays either the trees or the table.

"CS" Cell

In the "CS" cell, the weighted sum of the 3 outputs equals the weighted sum of the 5 inputs. In other words "a + b + c + d + e = 2 × h + 2 × g + f ". The "CS" cell is not only a "5Þ3 counter", but moreover the output "h" never depends on the input "e".

Carry propagation free adder

The "CS" cell does not propagate the input carry "e" to the output "h". It makes "carry propagation free" adders possible. The number of necessary "CS" cells is precisely given by the number of digits to be added.
Nevertheless, each digit is coded onto 2 bits and the digit value is the sum of those 2 bits. Therefore the possible digit values are '0', '1' and '2' .
  This notation system for integer numbers allows addition with a delay both short and independent of the digits number. Yet this system demands about twice as many bits as the standard binary notation for a comparable range. Consequently the same value may have several representations. Among the representations, the one with only '0' or '1' is unique. The vertical arrow next to the numbers value changes the representation without changing the value. The arrow "view" next to the button toggles the digits or bits display round.

Hybrid adder

We call "hybrid" the operators supporting different notations. The adder below, in some respects very simple, A is in binary, while B and S are both in "CS" notation.

Converter

Among the representations of A in "CS", the one with only '0' or '1' is unique. It is the binary represention of A. It is obtained by an adder S = A' + A".
  A fast adder like Sklansky's or Kogge et Stone's permits a fast conversion ; '0' gives 'K', '1' gives 'P' and '2' gives 'G' .