User space

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For the use of "user space" in Wikipedia, see Wikipedia:User page

A protected mode operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, device drivers and kernel extensions. In most operating systems, kernel memory is never swapped out to disk. In contrast, user space is the memory area where all user mode applications work and this memory can be swapped out when necessary. The term "userland" is often used for referring to operating system software that runs in user space.

Each user space process normally runs in its own virtual memory space, and, unless explicitly requested, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation. Depending on the privileges, processes can request the kernel to map part of another process's memory space to its own, which is necessary for debuggers. Programs can also request shared memory regions with other processes.

Another approach taken in experimental operating systems, is to have a single address space for all software, and rely on the programming language's virtual machine to make sure that arbitrary memory cannot be accessed — applications simply cannot acquire any references to the objects that they are not allowed to access.^[1] This approach has been implemented in JXOS, Unununium as well as Microsoft's Singularity research project.