memoryprotection

Memory protection is a hardware and operating‑system function that prevents a program from accessing memory it is not authorized to use. It is implemented by the memory management unit (MMU), which translates virtual addresses used by a process into physical addresses through a page table. Each page table entry contains protection bits that specify whether the page may be read, written, executed, or accessed by the processor. If a process attempts an illegal access, the MMU generates a page‑fault exception that the operating system handles, typically by terminating the offending process or by loading a required page from disk.

The primary goals of memory protection are system stability, security, and isolation. By keeping the memory spaces of different processes separate, the operating system ensures that a fault in one program, such as an accidental write or buffer overflow, does not corrupt data used by another program or the kernel itself. In multithreaded and multiuser environments, this isolation also supports security policies such as privilege separation, where kernel code runs in a higher privilege level than user code.

Memory protection can be implemented through segmentation, paging, or a combination of both. Segmentation assigns logical segments to processes with separate base and limit registers; paging divides memory into fixed‑size blocks managed by the MMU. Modern systems almost always use paging because it is efficient and cache‑friendly. Alongside memory protection, non‑executable memory (NX) and address‑space layout randomization (ASLR) enhance security by preventing executable code from residing in data pages and by randomizing the locations of useful memory regions.

Because memory protection is provided by both the processor hardware and the operating system, it is a fundamental feature of almost all contemporary computer systems, from embedded devices to large servers. Its design has evolved over decades, starting in early minicomputers and expanding into the complex virtual‑memory architectures that underpin today's multi‑user, multitasking operating systems.