A privilege level in the x86 instruction set controls the access of the program currently running on the processor to resources such as memory regions, I/O ports, and special instructions. There are 4 privilege levels ranging from 0 which is the most privileged, to 3 which is least privileged. Most modern operating systems use level 0 for the kernel/executive, and use level 3 for application programs. Any resource available to level n is also available to level 0..n, so the privilege levels are "rings".
It is not necessary to use all four privilege levels. Existing software that was designed to use only one or two levels of privilege can simply ignore the other levels offered by the 80386 and later processors. A one-level system should use privilege level zero; a two-level system should use privilege levels zero and three. Here level zero is called the Supervisor mode while level three is User mode. All versions of Windows below Windows XP use only the two-level system. The real mode programs in 8086 are executed at level 0 (highest privilege level) whereas virtual mode in 8086 executes all programs at level 3.
- Russinovich, Mark E.; David A. Solomon (2005). Microsoft Windows Internals (4 ed.). Microsoft Press. pp. 16. ISBN 978-0-7356-1917-3
- Sunil Mathur, "Microprocessor 8086: Architecture, Programming and Interfacing" , Eastern Economy Edition, PHI Learning
- Intel 80386 Programmer's Reference
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