kernelmode

Kernel mode, also known as supervisor mode or privileged mode, is a processor mode in which the executing code has unrestricted access to hardware and system resources. In most general-purpose operating systems, code runs in either kernel mode or user mode. The kernel and core services operate in kernel mode, while user applications run in user mode. Transitions between modes occur via traps, interrupts, or system calls.

In kernel mode, code can execute privileged instructions, configure memory management hardware, manage the interrupt controller, perform I/O operations, and manipulate kernel data structures. This level of access is required for core OS functions such as process scheduling, memory management, and device management. Attempts by user-mode code to execute privileged instructions or access restricted memory typically cause traps or faults.

Device drivers and many kernel subsystems run in kernel mode. A bug in kernel-mode code can crash the entire system or compromise security, which is why kernel integrity is a primary design concern. Some architectures and modern OS designs mitigate this risk by isolating or moving drivers to user space (as in certain microkernel architectures or driver-isolation schemes).

Architecture-specific details vary: many CPUs implement multiple privilege levels (for example, x86 uses ring 0 and ring 3), while ARM uses exception levels (EL1 for the kernel, EL0 for applications). In virtualization, a hypervisor may operate at an even higher privilege level than the guest kernel, adding another layer of separation.

Overall, kernel mode provides the essential, high-privilege execution context required for an operating system to manage hardware and core services, balanced against the need for stability and security.