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Concept# Lookahead carry unit

Summary

A lookahead carry unit (LCU) is a logical unit in digital circuit design used to decrease calculation time in adder units and used in conjunction with carry look-ahead adders (CLAs).
4-bit adder
A single 4-bit CLA is shown below:
16-bit adder
By combining four 4-bit CLAs, a 16-bit adder can be created but additional logic is needed in the form of an LCU.
The LCU accepts the group propagate (P_G) and group generate (G_G) from each of the four CLAs. P_G and G_G have the following expressions for each CLA adder:
:P_G = P_0 \cdot P_1 \cdot P_2 \cdot P_3
:G_G = G_3 + G_2 \cdot P_3 + G_1 \cdot P_2 \cdot P_3 + G_0 \cdot P_1 \cdot P_2 \cdot P_3
The LCU then generates the carry input for each CLA.
Assume that P_i is P_G and G_i is G_G from the ith CLA then the output carry bits are
:C_{4} = G_0 + P_0 \cdot C_0
:C_{8} = G_

Official source

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