CPU modes (also called processor modes, CPU states, CPU privilege levels and other names) are operating modes for the central processing unit of some computer architectures that place restrictions on the type and scope of operations that can be performed by certain processes being run by the CPU. This design allows the operating system to run with more privileges than application software.
Ideally, only highly-trusted kernel 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 system call 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).
In practice, however, system calls take time and can hurt the performance of a computing system, so it is not uncommon for system designers to allow some time-critical software (especially device drivers) to run with full kernel privileges.
Multiple modes can be implemented—allowing a hypervisor to run multiple operating system supervisors beneath it, which is the basic design of many virtual machine systems available today.
Read more about CPU Modes: Mode Types
Famous quotes containing the word modes:
“Men and women are brothers and sisters; they are not of different species; and what need be obtained to know both, but to allow for different modes of education, for situation and constitution, or perhaps I should rather say, for habits, whether good or bad.”
—Samuel Richardson (1689–1761)