Posts Tagged ‘cpu mode’

CPU Modes

CPU modes are operating mode for the CPU of some computer architecture that places restrictions on the type and scope of operations that can be performed by certain processes being run by the CPU. This design allows the OS to run with more privileges than application software.









system calls which need switch between user and kernel space take time and can hurt the performance of a computing system,it is not common to allow time-critical software to run with full kernel privileges.

Any CPU architecture supporting protected execution will offer two distinct operating mode:

  1. Kernel mode (ring 0)
  2. User mode (ring 3)

The hardware is aware of the current ring of the executing instruction thread at all time, thanks to special machine register.

The purpose of distinct operating modes for the CPU is to provide hardware protection against accidental or deliberate corruption of the system environment by software.

Only “trusted” portions of system software are allowed to execute in the unrestricted environment of kernel mode.

All other software executes in one or more user modes.

Microkernel OS attempt to minimize the amount of code running in privileged mode.

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