CPU modes

CPU modes (also called processor modes, CPU states, CPU privilege levels and other names) are operating modes for the <a href="/facts/Central_processing_unit/Jxp1bqMV">central processing unit</a> of most <a href="/facts/Computer_architecture/tHtwxgR2">computer architectures</a> that place restrictions on the type and scope of operations that can be performed by instructions being executed by the CPU. For example, this design allows an <a href="/facts/Operating_system/8XTuvMh2">operating system</a> to run with more privileges than <a href="/facts/Application_software/YQuj7Kog">application software</a> by running the operating systems and applications in different modes.
Ideally, only highly trusted <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernel</a> code is allowed to execute in the unrestricted mode; everything else (including non-supervisory portions of the operating system) runs in a restricted mode and must use a <a href="/facts/System_call/NW44AFJr">system call</a> (via <a href="/facts/Interrupt/Mm6C4rpc">interrupt</a>) to request the kernel perform on its behalf any operation that could damage or compromise the system, making it impossible for untrusted programs to alter or damage other programs (or the computing system itself). <a href="/facts/Device_driver/rDYnHU1G">Device drivers</a> are designed to be part of the kernel due to the need for frequent <a href="/facts/Input%2foutput/WQTE8Jas">I/O</a> access.
Multiple modes can be implemented, e.g. allowing a <a href="/facts/Hypervisor/3gX9fKlH">hypervisor</a> to run multiple operating system supervisors beneath it, which is the basic design of many <a href="/facts/Virtual_machine/BLevU3sx">virtual machine</a> systems available today.

CPU modes open-in-new

CPU modes