Adder Circuits
Computer Science: OCR A Level, topic 4.3.10
Adder Circuits
Adder circuits are used by the Arithmetic Logic Unit (ALU) in a processor to perform the addition of two binary digits.
Types of adder circuit
- Adder circuits come in two forms:
- Half adder circuits.
- Full adder circuits.
Half adder circuits
- Half adder circuits accept two inputs and output both the result and a carry bit.
- They make use of an XOR gate and an AND gate, laid out as shown.
Half adder table
- The actions of the two gates can be displayed in a table as shown.
- The output at S gives the 20 binary digit.
- The output at C gives the 21 binary digit (the carry bit).
Full adder circuits
- Full adders have the same output as a Half Adder (the sum and a carry bit), but they accept three inputs (two bits plus a carry bit).
- They are constructed from two Half Adders.
Full adder table
- In a full adder:
- S = A XOR B XOR C(in)
- C(out) = (A AND B) OR (C(in) AND (A XOR B))
Test yourself on Adder Circuits
Which binary operation is performed by adders?