The memory controller is a digital circuit that manages the flow of data going to and from the computer's main memory. A memory controller can be a separate chip or integrated into another chip, such as being placed on the same die or as an integral part of a microprocessor; in the latter case, it is usually called an integrated memory controller (IMC). A memory controller is sometimes also called a memory chip controller (MCC) or a memory controller unit (MCU). A common form of memory controller is the memory management unit (MMU) which in many operating systems implements virtual addressing. Most modern desktop or workstation microprocessors use an integrated memory controller (IMC), including microprocessors from Intel, AMD, and those built around the ARM architecture. Prior to K8 (circa 2003), AMD microprocessors had a memory controller implemented on their motherboard's northbridge. In K8 and later, AMD employed an integrated memory controller. Likewise, until Nehalem (circa 2008), Intel microprocessors used memory controllers implemented on the motherboard's northbridge. Nehalem and later switched to an integrated memory controller. Other examples of microprocessors that use integrated memory controllers include NVIDIA's Fermi, IBM's POWER5, and Sun Microsystems's UltraSPARC T1. While an integrated memory controller has the potential to increase the system's performance, such as by reducing memory latency, it locks the microprocessor to a specific type (or types) of memory, forcing a redesign in order to support newer memory technologies. When DDR2 SDRAM was introduced, AMD released new Athlon 64 CPUs. These new models, with a DDR2 controller, use a different physical socket (known as Socket AM2), so that they will only fit in motherboards designed for the new type of RAM. When the memory controller is not on-die, the same CPU may be installed on a new motherboard, with an updated northbridge.