1. A processor for use in a computer system, (a) the processor including processing means for processing instructions in at least three protection ring operating modes each associated with one of a hierarchy of privilege levels (b) said computer system including a memory means which includes a plurality of addressable storage locations for storing instructions requiring access to the memory locations, said memory means being comprised of plural groups of memory locations, wherein the number of memory locations in each memory group may vary, each memory group having an associated privilege means for identifying the protection ring operating modes in which the processor can access any memory location in the group, (c) said processor further comprising: A. means connected to said processing means and for connection to said memory means for iteratively retrieving instructions from the memory means requiring access to a location in memory; B. operating mode indicating means for identifying the privilege level of the current protection ring operating mode of said processor; C. virtual mode indicating means for indicating whether or not said processor is operating in a virtual mode; D. compression means connected to said operating mode indicating means for identifying a virtual mode operating mode in response to the current protection ring operating mode identified by said operating mode indicating means, said compression means operating in accordance with a compression function "F" which maps a set A=(0, 1,. .. , N) into a second set B=(0, 1,. .. , N), each element of the set A identifying one of the privilege levels when the processor is operating in the virtual mode and each element of the set B identifying one of the privilege levels when the processor is not operating in the virtual mode, the successive elements of each set corresponding to protection rings of progressively lower privilege, such that in each set "0" identifies the most privileged level and "N" identifies the least privileged level, said compression function F satisfying the following conditions: i. F (0)=1, and ii. If "i" is greater than "0" and less than or equal to "N", thnn F (i)=i; E. selection means connected to said operating mode indicating means, said virtual mode indicating means and said compression means for selectively transmitting as an output the privilege level identified by said compression means in response to said virtual mode indicating means indicating that said processor is operating in a virtual mode, and otherwise transmitting the privilege level identified by said operating mode indicating means; F. comparison means for comparing the output of said selection means to the privilege means of the location in memory to which the processor requires access to determine whether the processor can access the required memory location; and G. means responsive to a successful comparison by said comparison means for enabling the processing means to access the required memory location and execute the instruction.