86

Binary Incrementer

The increment micro-operation adds one binary value to the value of binary variables stored in a register. For instance, a 4-bit register has a binary value 0110, when incremented by one the value becomes 0111.

The increment micro-operation is best implemented by a 4-bit combinational circuit incrementer. A 4-bit combinational circuit incrementer can be represented by the following block diagram.

• A logic-1 is applied to one of the inputs of least significant half-adder, and the other input is connected to the least significant bit of the number to be incremented.
• The output carry from one half-adder is connected to one of the inputs of the next-higher-order half-adder.
• The binary incrementer circuit receives the four bits from A0 through A3, adds one to it, and generates the incremented output in S0 through S3.
• The output carry C4 will be 1 only after incrementing binary 1111.

Note: The 4-bit binary incrementer circuit can be extended to an n-bit binary incrementer by extending the circuit to include n half-adders. The least significant bit must have one input connected to logic-1. The other inputs receive the number to be incremented or the carry from the previous stage.