Digital computers perform various Arithmetic Building Blocks. The most basic operation, no doubt, is the addition of two binary digits. This simple addition consists of four possible elementary operations, namely,
The first three operations produce a sum whose length is one digit, but when the last operation is performed sum is two digits. The higher significant bit of this result is called a carry, and lower significant bit is called sum. The logic circuit which performs this operation is called a half-adder. The circuit which performs addition of three bits (two significant bits and a previous carry) is a full–adder. Let us see the logic circuits to perform half-adder and full-adder operations.