The central processing unit, commonly called the CPU or processor, is the integrated circuit that performs most of the calculations in a computer that the software requires. It is the most critical component in a computer, but it requires other systems to store, transport and utilize the data that it processes. The main connection between the CPU and the rest of the computer has evolved over the years as the technology has continued to progress.

System Bus

On early personal computers, the CPU was connected to the memory subsystem, input devices such as keyboards and serial ports, and output devices such as monitors and printers through a shared system bus. As CPUs became faster and more devices were used on the system bus, its shared nature led to bottlenecks in system performance and an alternative was developed.

Front-Side Bus

A dual-bus system was created with a back-side bus to connect the CPU to its cache and other CPUs, and a front-side bus to connect the CPU to the memory controller and the rest of the system, through the northbridge and southbridge controllers. The northbridge handled high-speed communications between the CPU, memory and high-performance expansion buses like PCI Express or AGP, while also communicating with the southbridge. The southbridge handled communications between slower devices, such as storage controllers, input devices, network communications and the PCI or ISA expansion buses. As technology further evolved, the speed of the front-side bus proved insufficient.

Point-to-Point Interconnect

To overcome the performance limitations of the front side-bus, CPU manufacturers began to incorporate the memory controller into the processor itself. New point-to-point interconnects replaced the front-side bus to provide a faster connection from the CPU to the rest of the system. AMD named its point-to-point interconnect HyperTransport, while Intel called theirs the Intel QuickPath Interconnect. Both operate at much faster speeds than the front-side bus, and with the memory controller located on the CPU, more bandwidth is available on the point-to-point interconnect for other system operations. The functions of the northbridge that are not integrated into the CPU have, in most cases, been combined with the functions of the southbridge to create a single input and output controller hub.

New Advances

Today's CPUs are including substantially more functions on the CPU die itself. Many of Intel's CPUs incorporate the functions of the northbridge, including the memory and PCI Express controllers, and they often include a graphics processor. The newer AMD CPUs include the memory controller, the PCI Express controller, a graphics processor, a media accelerator and an audio engine on the die. As further subsystems are integrated into the CPU, getting closer to a system-on-a-chip (SOC) design, the point-to-point interconnect and the rest of the system will likely be changed to meet the new requirements.