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
Other articles related to "cpu modes, cpu, mode, modes":
... The ARM architecture specifies the following CPU modes ... At any moment in time, the CPU can be in only one mode, but it can switch modes due to external events (interrupts) or programmatically ... User mode The only non-privileged mode ...
... The unrestricted mode is often called kernel mode, but many other designations exist (master mode, supervisor mode, privileged mode, supervisor state, etc.) ... Restricted modes are usually referred to as user modes, but are also known by many other names (slave mode, problem state, etc.) ... In kernel mode, the CPU may perform any operation allowed by its architecture any instruction may be executed, any I/O operation initiated, any area of ...
... Registers R0-R7 are the same across all CPU modes they are never banked ... R13 and R14 are banked across all privileged CPU modes except system mode ... That is, each mode that can be entered because of an exception has its own R13 and R14 ...
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“In the final analysis, style is art. And art is nothing more or less than various modes of stylized, dehumanized representation.”
—Susan Sontag (b. 1933)