Abstract
In processors and in digital circuit designs, adder is an important component. As a result, adder is the main area of research in VLSI system design for improving the performance of a digital system. The performance depends on power consumption and delay. Adders are not only used for arithmetic operations, but also for calculating addresses and indices. In digital design we have half adder and full adder, by using these adders we can implement ripple carry adder (RCA). RCA is used to perform any number of additions. In this RCA is serial adder and it has propagation delay problem. With increase in hard & fast circuits, delay also increases simultaneously. That’s the reason these Carry look ahead adders (CLA) are used. The carry look ahead adder speeds up the addition by reducing the amount of time required to determine carry bits. It uses two blocks, carry generator (Gi) and carry propagator (Pi) which finds the carry bit in advance for each bit position from the nearest LSB, if the carry is 1 then that position is going to propagate a carry to next adder.
Highlights
A ripple-carry adder works in the same way as pencil-and-paper methods of addition
Carry Look Ahead (CLA) design is based on the principle of looking at lower adder bits of argument and addend if higher orders carry generated
The architecture of Carry look ahead adders (CLA) is given in fig 5 and flow chart of CLA is given in fig 6
Summary
A ripple-carry adder works in the same way as pencil-and-paper methods of addition. It is possible that there may be a carry out of this digit position (for example, in pencil-and-paper methods, "6 + 6 = 2, carry 1"). 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 position to the right. Carry-look ahead depends on two things: 1) Calculating for each digit position whether that position is going to propagate a carry if one comes in from the right. 2) Combining these calculated values to be able to deduce quickly whether, for each group of digits, that group is going to propagate a carry that comes in from the right.
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