A memory management unit (MMU) is an integrated circuit that supports virtual memory and translating virtual addresses into physical addresses through a process called memory paging. Hence, they can also be referred to as a paged memory management unit (PMMU).^[1]

Operation

The virtual address space (the range of addresses used by the processor) is divided into pages, whose size is measured by a power of 2, and can be as small as a few kilobytes. 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 contains a page table which is indexed (possibly associatively) by the page number. Each page table entry (PTE) gives the physical page number corresponding to the virtual one. This is combined with the page offset to give the complete physical address.

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

It is possible that no physical memory (RAM) has been allocated to a given virtual page, in which case the MMU will signal a "page fault" to the CPU. The operating system will then try to find a spare page 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, using some replacement algorithm, and save it to disk (memory paging). There may also be a shortage of PTEs, in which case the OS will have to free one for the new mapping.

In a multitasking system, all processes compete for the use of memory and of the MMU. Some memory management architectures allow each process to have its own area or configuration of the page table, with a mechanism to switch between different mappings on a process switch. This means that all processes can have the same virtual address space rather than require load-time relocation.

An MMU also solves 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.^[1]

Processor integration

In early designs, memory management was performed by a separate integrated circuit, such as the Motorola 68851 which was optionally used with the Motorola 68020 processor in the Macintosh II, or the Z8015 which was used with the Zilog Z80 family of processors. Later processors, such as the Motorola 68030, Zilog Z280, and Intel i386 have memory management units built directly onto the same die as the CPU.^[1]

The first Mac model to have MMU capabilities built in by default was the 68030-based Macintosh IIx, released in September 1988. All modern Macs now have this capability built into the processor.^[2]

References

External links

• Memory management unit at Wikipedia

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