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A carry-lookahead adder CLA or fast adder is a type of adder used in digital logic. A carry-lookahead adder improves speed by reducing the amount of time required to determine carry bits. It can be contrasted with the simpler, but usually slower, ripple-carry adder RCAfor which the carry bit is calculated alongside the sum bit, and each bit must wait until the previous carry bit have been calculated to begin calculating its own result and carry bits.
The carry-lookahead adder calculates one or more carry bits before the sum, which reduces the wait time to calculate the result of the larger-value bits of the adder. Charles Babbage recognized the performance penalty imposed by ripple-carry and developed mechanisms for anticipating carriage in his computing engines. Rosenberger of IBM filed for a patent on a modern binary carry-lookahead adder in A ripple-carry adder works in the same way as pencil-and-paper methods of addition.
Starting at the rightmost least significant digit position, the two corresponding digits are added and a result obtained. Accordingly, all digit positions other than the rightmost one need to take into account the possibility of having to add an extra 1 from a carry that has come in from the next position to the right.
This means that no digit position can have an absolutely final value until it has been established whether or not a carry is coming in from the right. Moreover, if the sum without a carry is 9 in pencil-and-paper methods or 1 in binary arithmeticit is not even possible to tell whether or not a given digit position is going to pass on a carry to the position on its left.
It is the "rippling" of the carry from right to left that gives a ripple-carry adder its name, and its slowness. When adding bit integers, for instance, allowance has to be made for the possibility that a carry could have to ripple through every one of the 32 one-bit adders. The net effect is that the carries start by propagating slowly through each 4-bit group, just as in a ripple-carry system, but then move four times as fast, leaping from one lookahead-carry unit to the next.
Finally, within each group that receives a carry, the carry propagates slowly within the digits in that group. The more bits in a group, the more complex the lookahead carry logic becomes, and the more time is spent on the "slow roads" in each group rather than on the "fast road" between the groups provided by the lookahead carry logic. On the other hand, the fewer bits there are in a group, the more groups have to be traversed to get from one end of a number to the other, and the less acceleration is obtained as a result.
Deciding the group size to be governed by lookahead carry logic requires a detailed analysis of gate and propagation delays for the particular technology being used. It is possible to have more than one level of lookahead-carry logic, and this is in fact usually done.
Each lookahead-carry unit already produces a signal saying "if a carry comes in from the right, I will propagate it to the left", and those signals can be combined so that each group of, say, four lookahead-carry units becomes part of a "supergroup" governing a total of 16 bits of the numbers being added. The "supergroup" lookahead-carry logic will be able to say whether a carry entering the supergroup will be propagated all the way through it, and using this information, it is able to propagate carries from right to left 16 times as fast as a naive ripple carry.
With this kind of two-level implementation, a carry may first propagate through the "slow road" of individual adders, then, on reaching the left-hand end of its group, propagate through the "fast road" of 4-bit lookahead-carry logic, then, on reaching the left-hand end of its supergroup, propagate through the "superfast road" of bit lookahead-carry logic.
Again, the group sizes to be chosen depend on the exact details of how fast signals propagate within logic gates and from one logic gate to another. For very large numbers hundreds or even thousands of bitslookahead-carry logic does not become any more complex, because more layers of supergroups and supersupergroups can be added as necessary. The increase in the number of gates is also moderate: However, the "slow roads" on the way to the faster levels begin to impose a drag on the whole system for instance, a bit adder could have up to 24 gate delays in its carry processingand the mere physical transmission of signals from one end of a long number to the other begins to be a problem.
At these sizes, carry-save adders are preferable, since they spend no time on carry propagation at all. Carry-lookahead logic uses the concepts of generating and propagating carries. Although in the context of a carry-lookahead adder, it is most natural to think of generating and propagating in the context of binary addition, the concepts can be used more generally than this.
In the descriptions below, the word digit can be replaced by bit when referring to binary addition of 2. The addition of two 1-digit inputs A and B is said to generate if the addition will always carry, regardless of whether there is an input-carry equivalently, regardless of whether any less significant digits in the sum carry. The addition of two 1-digit inputs A and B is said to propagate if the addition will carry whenever there is an input carry equivalently, when the next less significant digit in the sum carries.
Note that propagate and generate are defined with respect to a single digit of addition and do not depend on any other digits in the sum. Sometimes a slightly different definition of propagate is used. Due to the way generate and propagate bits are used by the carry-lookahead logic, it doesn't matter which definition is used.
In the case of binary addition, this definition is expressed by. For binary arithmetic, or is faster than xor and takes fewer transistors to implement. Given these concepts of generate and propagate, a digit of addition carries precisely when either the addition generates or the next less significant bit carries and the addition propagates.
For each bit in a binary sequence to be added, the carry-lookahead logic will determine whether that bit pair will generate a carry or propagate a carry. This allows the circuit to "pre-process" the two numbers being added to determine the carry ahead of time.
Then, when the actual addition is performed, there is no delay from waiting for the ripple-carry effect or time it takes for the carry from the first full adder to be passed down to the last full adder.
Below is a simple 4-bit generalized carry-lookahead circuit that combines with the 4-bit ripple-carry adder we used above with some slight adjustments:. For the example provided, the logic for the generate g and propagate p values are given below. The numeric value determines the signal from the circuit above, starting from 0 on the far left to 3 on the far right:. To determine whether a bit pair will propagate a carry, either of the following logic statements work:.
The XOR is used normally within a basic full adder circuit; the OR is an alternative option for a carry-lookahead onlywhich is far simpler in transistor-count terms. The carry-lookahead 4-bit adder can also be used in a higher-level circuit by having each CLA logic circuit produce a propagate and generate signal to a higher-level CLA logic circuit.
Putting 4 4-bit CLAs together yields four group propagates and four group generates. The calculation of the gate delay of a bit adder using 4 CLAs and 1 LCU is not as straight forward as the ripple carry adder.
The Manchester carry chain is a variation of the carry-lookahead adder  that uses shared logic to lower the transistor count. As can be seen above in the implementation section, the logic for generating each carry contains all of the logic used to generate the previous carries. A Manchester carry chain generates the intermediate carries by tapping off nodes in the gate that calculates the most significant carry value.
However, not all logic families have these internal nodes, CMOS being a major example. Dynamic logic can support shared logic, as can transmission gate logic. One of the major downsides of the Manchester carry chain is that the capacitive load of all of these outputs, together with the resistance of the transistors causes the propagation delay to increase much more quickly than a regular carry lookahead.