The MMU, which stands for Memory Management Unit, is a component in CPUs that handles memory access and management. Its main functions are to translate virtual addresses to physical addresses through paging and segmentation, allocate memory dynamically to programs, and protect memory spaces from unauthorized access.
One of the primary functions of the MMU is to handle address translation between virtual addresses and physical addresses. Programs generate virtual addresses when accessing memory, but the actual physical memory locations are accessed through physical addresses. The MMU maps virtual addresses to physical addresses so that programs can access the correct locations in physical memory.
There are two main techniques used for address translation:
- Paging – Memory is divided into fixed-size blocks called pages. The MMU stores page table mappings to translate virtual page numbers to physical page numbers.
- Segmentation – Memory is divided into variable-sized segments. Segment descriptors define the mapping between virtual addresses and physical addresses.
Without the MMU handling address translation, programs would have to interface directly with physical memory which is complex and inflexible. Virtual addressing enabled by the MMU makes memory access simpler and more efficient.
Dynamic Memory Allocation
The MMU allows memory to be dynamically allocated to programs as needed. This means that not all the memory needs to be assigned to programs in advance. Instead, the program can request memory as required which the MMU will allocate in available spaces in physical memory.
Dynamic memory allocation requires:
- Paging/segmentation – To map newly allocated memory blocks to virtual address spaces.
- Unused physical memory areas – For allocating to programs when requested.
- Memory tracking – The MMU tracks used and unused memory blocks.
The MMU may use techniques like paging, segmentation and swapping to memory and disks to optimize memory allocation. This provides much more flexibility and efficient utilization of memory.
The MMU provides access control and memory protection to ensure programs only access memory they are authorized to. This prevents malicious or buggy programs from accidentally or intentionally modifying memory belonging to other programs or the operating system.
The MMU enforces protection through:
- Access permission checks – Each page or segment descriptor includes read/write/execute permissions.
- Address bounds checking – Virtual address bounds are checked against allocated memory blocks.
- Execution prevention – Data pages are marked non-executable.
The OS configures the access permissions in the MMU page tables and descriptors. Any access violation will trigger an exception handled by the OS. This isolates programs and protects memory integrity.
Caching and Buffering
The MMU may incorporate Fast Translation Lookaside Buffers (TLBs) that function as caches to optimize address translation. The TLBs store subsets of page table and segment mappings for faster lookup. This avoids having to access the full page tables in memory for every translation.
TLBs improve performance by reducing the latency of memory accesses. However, their smaller size also leads to possible misses if the requested translation is not cached. Operating systems try to optimize TLB hit rates through page allocation policies optimizing locality of access.
The MMU may also include prefetching buffers to request memory access ahead of time and hide memory latency. Buffering helps smooth out bursts in memory traffic as well.
The MMU enables the use of virtual memory in operating systems. Virtual memory is an illusion of large contiguous memory space created using demand paging of physical memory and disk swapping. The large virtual address space is managed by the MMU mapping pages to physical memory or disk as needed.
Virtual memory provides several benefits:
- Overcommitment of memory – Virtual address space can exceed physical memory.
- Process isolation – Unique virtual address spaces for each process.
- Memory protection – Access controlled through MMU mappings.
- Dynamic memory – Pages loaded to physical RAM on demand.
The MMU handles mapping virtual pages to physical frames transparently using page tables. This allows the OS to efficiently implement virtual memory. The illusion of larger memory capacities is maintained despite actual physical memory constraints.
The MMU also plays a role during context switches when the CPU changes from one program to another. The OS has to change the MMU address translation mappings on each switch.
For example, in a page table system, the OS would swap to the new program’s page table. The TLBs may also need to be flushed to prevent old invalid mappings being used. Segment descriptor tables similarly need to be swapped.
Hardware support in the MMU makes context switching faster. The process state including registers, program counters and address mappings can be restored quicker. This reduces latency and overhead when switching programs.
MMU Design Considerations
Some additional considerations in the MMU design include:
- Granularity – Larger page or segment sizes reduce size of mappings but reduce allocation flexibility.
- Address space – 32-bit, 36-bit, 64-bit architectures have different address space sizes.
- Caching – Bigger TLB caches increase hit rate but take longer to search through.
- Multithreading – Multiple thread contexts may require duplicating or partitioning TLBs.
- Virtualization – Nested page tables can map guest virtual addresses to host physical addresses.
Choosing the right tradeoffs requires understanding the memory access patterns and usage. The MMU design also needs to scale across CPU generations as address sizes increase.
Some examples of errors that may occur related to the MMU include:
- Segmentation fault – Accessing invalid or unmapped virtual address.
- Bus error – Physical address not matching any device.
- TLB miss – Missing translation lookup in TLB cache.
- Page fault – Triggered page swap to disk.
- Access violation – Process lacks required permissions.
The OS is usually responsible for handling these errors and determining the appropriate response. For example, a page fault will require loading the missing page containing the data.
In summary, the MMU plays a critical role in memory access and virtual memory management in modern computer systems. The dynamic address translation, allocation, protection and virtualization capabilities enabled by the MMU allow more efficient, safe and flexible use of memory. As memory demands increase, the capabilities supplied by the MMU will continue evolving to meet future requirements.