CPUmodus

CPUmodus is a term used to describe the operational states, or modes, of a central processing unit (CPU) in modern computer systems. The word is a portmanteau of CPU and modus, common in German-language discussions of computer architecture. CPUmodus encompasses the privilege levels and architectural states that govern which instructions may be executed and what hardware resources may be accessed by running code.

The core distinction is typically between privileged modes, where full access to hardware and system control is granted, and non-privileged or user modes, where such access is restricted. Many CPU families also include specialized modes for virtualization (hypervisor mode), firmware or system management tasks (system management mode), and legacy or architectural states such as real mode, protected mode, or long mode. In architectures like ARM, a comparable hierarchy exists with privileged and non-privileged execution, plus monitor and secure modes in certain variants.

Mode transitions are triggered by events such as system calls, interrupts, traps, or virtualization exits and entries. The CPU enforces transitions by changing privilege levels, saving and restoring context, and controlling access to memory via page tables and permission bits. These mechanisms are central to operating system design, security, and stability, because they determine how user applications interact with the hardware and kernel services.

Impact and usage: CPUmodus concepts inform OS kernels, hypervisors, and firmware interfaces. They also influence power management, where certain modes enable lower power states, and performance features, where mode switches incur overhead but enable protection and isolation. See also topics such as privileged instruction, protection rings, real/protected/long modes, and virtualization.