Or far return parameters are released from the stack

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Unformatted text preview: Services (Device Drivers, Etc.) Applications Highest 0 Lowest 3 Level 0 Level 1 Level 2 Level 3 1 2 Privilege Levels Figure 6-3. Protection Rings In this example, the highest privilege level 0 (at the center of the diagram) is used for segments that contain the most critical code modules in the system, usually the kernel of an operating system. The outer rings (with progressively lower privileges) are used for segments that contain code modules for less critical software. Code modules in lower privilege segments can only access modules operating at higher privilege segments by means of a tightly controlled and protected interface called a gate. Attempts to access higher privilege segments without going through a protection gate and without having sufficient access rights causes a general-protection exception (#GP) to be generated. If an operating system or executive uses this multilevel protection mechanism, a call to a procedure that is in a more privileged protection level than the calling procedure is handled in a similar manner as a far call (see Section 6.3.2, "Far CALL and RET Operation"). The differences are as follows: The segment selector provided in the CALL instruction references a special data structure called a call gate descriptor. Among other things, the call gate descriptor provides the following: -- access rights information -- the segment selector for the code segment of the called procedure -- an offset into the code segment (that is, the instruction pointer for the called procedure) Vol. 1 6-9 PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS The processor switches to a new stack to execute the called procedure. Each privilege level has its own stack. The segment selector and stack pointer for the privilege level 3 stack are stored in the SS and ESP registers, respectively, and are automatically saved when a call to a more privileged level occurs. The segment selectors and stack pointers for the privilege level 2, 1, and 0 stacks are stored in a system segment called the task state segment (TSS). The use of a call gate and the TSS during a stack switch are transparent to the calling procedure, except when a general-protection exception is raised. 6.3.6 CALL and RET Operation Between Privilege Levels When making a call to a more privileged protection level, the processor does the following (see Figure 6-4): 1. Performs an access rights check (privilege check). 2. Temporarily saves (internally) the current contents of the SS, ESP, CS, and EIP registers. Stack for Calling Procedure Stack for Called Procedure Stack Frame Before Call Param 1 Param 2 Param 3 ESP Before Call ESP After Call Calling SS Calling ESP Param 1 Param 2 Param 3 Calling CS Calling EIP Stack Frame After Call ESP After Return Param 1 Param 2 Param 3 ESP Before Return Calling SS Calling ESP Param 1 Param 2 Param 3 Calling CS Calling EIP Note: On a return, parameters are released on both stacks based on the optional n operand in the RET n instruction. Figure 6-4....
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