In order to implement virtual memory, it is necessary for the computer system to have special memory management hardware. This hardware is often known as an MMU (Memory Management Unit).
Without an MMU, when the CPU accesses RAM, the actual RAM locations never change — memory address 123 is always the same physical location within RAM.
A memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks.
A MMU is responsible for translating logical memory into a physical memory address, and for performing checks to make sure the translation can be done and that the access is permitted.