A Level Computer Science OCR Practice Exam

Which of the following statements is true regarding a Half Adder?

1. It has three inputs and two outputs

2. It performs subtraction operations

3. It produces a digit bit and a carry bit

4. It can store multiple bits of data

The correct answer is: It produces a digit bit and a carry bit

A Half Adder is a fundamental digital circuit designed primarily for the purpose of adding two single binary digits (bits). The two inputs to a Half Adder represent the two bits to be added (let's call them A and B). When these two bits are added together, the output consists of two significant parts: the sum and the carry. The sum output represents the least significant bit of the result, while the carry output indicates whether or not there is an overflow into the next higher bit position. Specifically, if both input bits A and B are 1, the result would be a sum of 0 with a carry of 1, indicating that the actual binary result of the addition will require consideration of the next bit. Given this fundamental function, the correct statement emphasizes the basic capability of the Half Adder to generate a distinct sum bit alongside a carry bit, while not implying any ability to handle three inputs, perform subtraction, or store multiple bits of data, as those aspects are typically characteristics of more complex circuits or memory elements. Thus, the assertion that it produces a digit bit and a carry bit accurately captures the essence of the Half Adder's functionality.