Memory Management Unit - How IT Works

How It Works

Modern MMUs typically divide the virtual address space (the range of addresses used by the processor) into pages, each having a size which is a power of 2, usually a few kilobytes, but they may be much larger. The bottom n bits of the address (the offset within a page) are left unchanged. The upper address bits are the (virtual) page number. The MMU normally translates virtual page numbers to physical page numbers via an associative cache called a translation lookaside buffer (TLB). When the TLB lacks a translation, a slower mechanism involving hardware-specific data structures or software assistance is used. The data found in such data structures are typically called page table entries (PTEs), and the data structure itself is typically called a page table. The physical page number is combined with the page offset to give the complete physical address.

A PTE or TLB entry may also include information about whether the page has been written to (the dirty bit), when it was last used (the accessed bit, for a least recently used page replacement algorithm), what kind of processes (user mode, supervisor mode) may read and write it, and whether it should be cached.

Sometimes, a TLB entry or PTE prohibits access to a virtual page, perhaps because no physical random access memory has been allocated to that virtual page. In this case the MMU signals a page fault to the CPU. The operating system (OS) then handles the situation, perhaps by trying to find a spare frame of RAM and set up a new PTE to map it to the requested virtual address. If no RAM is free, it may be necessary to choose an existing page (known as a victim), using some replacement algorithm, and save it to disk (this is called "paging"). With some MMUs, there can also be a shortage of PTEs or TLB entries, in which case the OS will have to free one for the new mapping.

In some cases a "page fault" may indicate a software bug. A key benefit of an MMU is memory protection: an OS can use it to protect against errant programs, by disallowing access to memory that a particular program should not have access to. Typically, an OS assigns each program its own virtual address space.

An MMU also reduces the problem of fragmentation of memory. After blocks of memory have been allocated and freed, the free memory may become fragmented (discontinuous) so that the largest contiguous block of free memory may be much smaller than the total amount. With virtual memory, a contiguous range of virtual addresses can be mapped to several non-contiguous blocks of physical memory.

In some early microprocessor designs, memory management was performed by a separate integrated circuit such as the VLSI VI475 or the Motorola 68851 used with the Motorola 68020 CPU in the Macintosh II or the Z8015 used with the Zilog Z8000 family of processors. Later microprocessors such as the Motorola 68030 and the Zilog Z280 placed the MMU together with the CPU on the same integrated circuit, as did the Intel 80286 and later x86 microprocessors.

While this article concentrates on modern MMUs, commonly based on pages, early systems used a similar concept for base-limit addressing, that further developed into segmentation. Those are occasionally also present on modern architectures. The x86 architecture provided segmentation rather than paging in the 80286, and provides both paging and segmentation in the 80386 and later processors (although the use of segmentation is not available in 64-bit operation).