Editing Operating system (section)

====Memory management====
{{Main|Memory management}}

Among other things, a multiprogramming operating system [[kernel (operating system)|kernel]] must be responsible for managing all system memory which is currently in use by the programs. This ensures that a program does not interfere with memory already in use by another program. Since programs time share, each program must have independent access to memory.

Cooperative memory management, used by many early operating systems, assumes that all programs make voluntary use of the [[kernel (operating system)|kernel]]'s memory manager, and do not exceed their allocated memory. This system of memory management is almost never seen anymore, since programs often contain bugs which can cause them to exceed their allocated memory. If a program fails, it may cause memory used by one or more other programs to be affected or overwritten. Malicious programs or viruses may purposefully alter another program's memory, or may affect the operation of the operating system itself. With cooperative memory management, it takes only one misbehaved program to [[Crash (computing)|crash]] the system.

[[Memory protection]] enables the [[kernel (operating system)|kernel]] to limit a process' access to the computer's memory. Various methods of memory protection exist, including [[memory segmentation]] and [[paging]]. All methods require some level of hardware support (such as the [[80286]] MMU), which does not exist in all computers.

In both segmentation and paging, certain [[protected mode]] registers specify to the CPU what memory address it should allow a running program to access. Attempts to access other addresses trigger an interrupt, which causes the CPU to re-enter [[supervisor mode]], placing the [[kernel (operating system)|kernel]] in charge. This is called a [[segmentation violation]] or Seg-V for short, and since it is both difficult to assign a meaningful result to such an operation, and because it is usually a sign of a misbehaving program, the [[kernel (operating system)|kernel]] generally resorts to terminating the offending program, and reports the error.

Windows versions 3.1 through ME had some level of memory protection, but programs could easily circumvent the need to use it. A [[general protection fault]] would be produced, indicating a segmentation violation had occurred; however, the system would often crash anyway.