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.
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... There are few satisfactory resolutions to this issue ... It is usually not possible to modify the program as source code is typically not available and there is no room in the instruction stream to introduce a STI without massive editing at the assembly level ...
... That selector consists of a 2-bit Requested Privilege Level (RPL), a 1-bit Table Indicator (TI), and a 13-bit index ... It then performs the privilege check max(CPL, RPL) ≤ DPL where CPL is the current privilege level (found in the lower 2 bits of the CS register), RPL is the requested privilege level from the segment ... All privilege levels are integers in the range 0–3, where the lowest number corresponds to the highest privilege ...
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