5.8 Access Control

5.8 Access Control

1. In Code Segements

   • Machine instructions can cause a transfer of execution
     • If a transfer occurs within the current code it is subject to limit checking
       • Limit Checking = the processor checks that the destination of the instruction does not exceed the limit of the current code segment
     • If the transfer happens outside of the segment they are subject to privilege checks by the processor
       • This can happen through the use of gates or if the process selects the descriptor of another segment
         • If another segment is chosen the transfer is permitted if the CPL is the same as the DPL of the destination code segment

2. Conforming Code Segments

   • CPL may be less privileged than DPL if the current code segment is a conforming code segment (this would be indicated in the segment descriptor
     • Hence a conforming segment runs at the privilege level of the calling procedure rather than the actual code segment
       • Conforming code segments are used for programs which do not require access to protected system facilities
         • When control is transferred to a conforming segment the CPL does not change