Memory Management Unit (MMU)
A Memory Management Unit (MMU) is a critical component of computer architecture, primarily responsible for handling memory and cache operations between the central processing unit (CPU) and main memory. It facilitates a variety of tasks including memory protection, virtual memory management, and address translation.
Functionality
Address Translation
The MMU translates virtual addresses generated by the CPU into physical addresses in the hardware's main memory. This allows the CPU to operate as though it has access to a large, contiguous block of memory, even though it may be physically dispersed across many locations. This is crucial for implementing virtual memory and ensuring process isolation and memory protection.
Memory Protection
Memory protection prevents processes from accessing memory that has not been allocated to them. The MMU checks memory accesses to ensure they are within the bounds of the allocated memory for a given process. This prevents a faulty or malicious process from corrupting the memory space of another process, thus maintaining system stability and security.
Virtual Memory Management
The MMU plays an essential role in implementing virtual memory, allowing systems to use more memory than what is physically available by using disk storage as an extension of RAM. This makes programs perceive they are working with large contiguous blocks of free memory, while in reality, data is fetched from physical memory or swapped out to disk as needed.
Components and Operation
Translation Lookaside Buffer (TLB)
A key component of the MMU is the Translation Lookaside Buffer (TLB), a cache that stores recent translations of virtual memory to physical memory addresses to speed up the translation process and improve performance.
Segmentation and Paging
MMUs can use both segmentation and paging to manage memory. Segmentation divides memory into different segments based on type, while paging divides memory into fixed-size blocks or pages. These techniques can be used individually or together to provide a flexible and efficient memory management system.
Special Forms
Input-Output Memory Management Unit (IOMMU)
In some systems, an Input-Output Memory Management Unit (IOMMU) connects a direct-memory-access (DMA)-capable I/O bus to the main memory. It allows hardware devices to access physical memory, enabling efficient data transfer directly between devices and memory without CPU intervention.
Memory Protection Unit (MPU)
Simpler systems might employ a Memory Protection Unit (MPU), which offers only memory protection without full virtual memory support. It is a streamlined variant of the MMU, crucial in systems where memory safety is paramount but virtual memory's flexibility is unnecessary.
Historical Context
Historically, MMUs were implemented as separate integrated circuits. Notable examples include:
- The Motorola 68851, used with the Motorola 68020 CPU, implemented in systems like the Apple Macintosh II.
- The VLSI Technology VI475 and Zilog Z8010, which were used with their respective processor families.
Modern microprocessors integrate MMU functionality directly into the CPU, enhancing speed and reducing total system costs.