Memory Management Unit

(M-M-U)

Originally any device or circuit that implemented virtual memory and paging (i.e., translating virtual memory addresses into physical addresses); now, MMUs are usually part of a CPU or a circuit on the same chip as the CPU.

Virtual memory is usually divided into pages. As memory pages are used the MMU develops a collection of page table entries (PTE) which specify the physical page being referenced along with page offsets that define physical addresses. The table may also contain information about when the page was last accessed and security information about who can read/write to the page. If a virtual page is accessed that has not been assigned a physical page, the MMU will take care of that; saving pages to disk using a caching algorithm if no additional RAM is available to assign. As memory becomes fragmented due to this use and freeing of pages the MMU may "garbage collect" and reassign memory blocks in a more contiguous manner to free up larger blocks.

More Information

• Class Notes
• Memory Management Reference