The Memory Management Unit is a unit that sits between the memory and the processor in most modern computers. It's job is to virtualize the memmory, generating an interrupt if the current aplication exceeds its limitations set up by the OS. Most modern OSes requeres a MMU (To protect aplications from each other), however, while most desktop computers do have MMUs, some embedded systems does not. Examples of MMU-less systems include the Dragonball processor (MC 68000-familly) used in the Palm Pilot.