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Experiment 6 - The n-bit Ripple-Carry Adder

A 3-bit adder can only calculate its result after the carry bit of the lower digits are known. The execution time of a 3-bit ripple-carry adder is therefore 5 × τ = 300 ns! (see illustration).

Illustration delay of a 3-bit ripple-carry adder.

Each additional digit must again wait for the result of all the lower digits. The execution time of a ripple-carry adder of two 8-bit numbers would already be 900 ns!


Circuit symbol full adder.

Circuit diagram (n+1)-bit ripple-carry adder.

Modern microprocessors have arithmetic logic units that handle 64-bit numbers. An addition of two 64-bit numbers with a ripple adder-carry would take an eternity compared to other execution times of the microprocessor.

Therefore, no ripple-carry adders are used in modern microprocessors, but so-called carry-lookahead adders...

Formula Execution Time

You calculate the total execution time Ttotal of a n-bit ripple-carry adder with...

Ttotal = (2n - 1) τ

τ : Gate delay time

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