UserMode

User mode, also written as user-mode or usermode, is the CPU state in which standard application software executes with limited privileges within an operating system. In contrast to kernel mode, where the kernel can execute privileged instructions and access all hardware, user mode provides isolation and protection by restricting what a running program may do directly.

Within user mode, each process typically runs in its own virtual address space, protected from interference by other processes. When a program needs to perform an operation that requires privileged access (such as accessing hardware, creating files, or communicating with the kernel), it makes a system call or uses an operating-system API. The processor traps to the kernel, the OS code runs in kernel mode to service the request, and control returns to user mode when finished.

CPU architectures typically implement privilege levels (for example, rings on x86 or supervisor mode on ARM). The separation helps prevent a faulty or malicious program from corrupting the system or reading other processes’ memory. The kernel enforces this boundary, schedules processes, and handles interrupts and exceptions.

Device drivers and many kernel services run in kernel mode, while user-mode programs interact with them through controlled interfaces such as system calls, devices, or libraries. Some environments also provide user-space implementations of OS services for testing, debugging, or containment, such as User-Mode Linux, which runs Linux itself as a user-space process.

See also: kernel mode, system call, virtual memory, user-space, User-Mode Linux.