The central processing unit (CPU) is the principal component for executes instructions . The CPU is often called the heart of the computer for this reason. In modern computer systems, a good of deal processing is however performed external to the CPU, in graphics cards, peripheral controllers etc. It is also typical in modern systems to have more than one CPU. Where these addition CPUs share a common integrated circuit (chip) the term multiple cores is used. So for example, a quad core refers to four CPUs on a single chip.