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Designing SOCs with Configured Cores by Steve Leibson

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9.6. Privilege Levels and Rings

The simplest sort of privilege mechanism, implemented by many CPUs, employs two privilege levels. These two levels are often called the user and supervisor or user and kernel levels. The MMU makes higher-level instruction and data spaces inaccessible to user-level code to protect the most privileged (supervisor or kernel) code and data.

The Diamond 232L CPU’s MMU operates with four privilege levels called rings. In the Diamond 232L CPU, kernel code resides at Ring 0. Code running at Ring 0 can access the address spaces in all of the other privilege rings. The privilege levels are called rings because conceptually, lower-numbered levels (with higher privilege) envelop or “ring” higher-numbered levels. Figure 9.7 ...

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