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. |
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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... |
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