memorymapped

Memorymapped refers to a technique in which a region of a process’s virtual address space is mapped directly to another resource, such as a file, a device, or a shared memory region. This mapping allows programs to access the resource using ordinary memory read and write operations rather than explicit I/O system calls, enabling potential performance and simplicity improvements.

In a memory-mapped system, the operating system uses the virtual memory unit and page tables to associate blocks of virtual addresses with corresponding blocks of the backing resource. When a program touches a page for the first time, a page fault may occur, causing the OS to fetch data from the file or device into physical memory. Writes may be cached and later synchronized back to the backing store, depending on the mapping mode and policies. Typical synchronization mechanisms include explicit flush operations or automatic synchronization on program termination.

There are two common forms of memory-mapped access. Memory-mapped files map the contents of a file into the process’s address space, enabling fast random access, efficient file I/O, and straightforward interprocess sharing when shared mappings are used. Memory-mapped I/O maps device memory into the address space, allowing software to interact with hardware registers or buffers as if they were regular memory, which can reduce the overhead of traditional I/O calls but may require constraints on caching, ordering, and protection.

Advantages of memory mapping include potential performance gains from reduced copying, simplified data access patterns, and convenient interprocess communication. Disadvantages include consumption of virtual address space, potential cache and coherence complexities, and portability concerns across systems or hardware. Common implementations include POSIX mmap and Windows memory-mapped files APIs, with platform-specific nuances in behavior and synchronization.