![]() But depending on how the system call was implemented, an application might be able to invoke the system call with a parameter of an address within segment X. Access to the segment is only allowed if both of the following are true: When accessing a segment, there are actually two checks that must be performed. Every memory access (implicitly 2 or otherwise) uses a segment selector as part of the access. A segment selector is just a 16-bit value that references a segment. RPL is a privilege level associated with a segment selector. So: To access a segment, CPL must be less than or equal to the DPL of the segment.Privilege levels range from 0-3 lower numbers are more privileged.It defines the minimum 1 privilege level required to access the segment. The DPL is the privilege level of a segment.The CPL is your current privilege level.To make this simpler, let's first just consider CPL and DPL:
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