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Call gates for these procedures can be set up that

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Unformatted text preview: lowered to the DPL of the destination code segment and a stack switch occurs (refer to Section 4.8.5., “Stack Switching”). If a call or jump is made to a more privileged conforming destination code segment, the CPL is not changed and no stack switch occurs. Code Segment A CPL=3 Gate Selector A RPL=3 Gate Selector B3 RPL=3 Call Gate A DPL=3 3 Lowest Privilege Code Segment B CPL=2 Gate Selector B1 RPL=2 Call Gate B DPL=2 2 Code Segment C CPL=1 Gate Selector B2 RPL=1 No Stack Switch Occurs Stack Switch Occurs Code Segment E DPL=0 Nonconforming Code Segment 1 Code Segment D DPL=0 0 Highest Privilege Conforming Code Segment Figure 4-10. Example of Accessing Call Gates At Various Privilege Levels 4-20 PROTECTION Call gates allow a single code segment to have procedures that can be accessed at different privilege levels. For example, an operating system located in a code segment may have some services which are intended to be used by both the operating system and application software (such as procedures for handling character I/O). Call gates for these procedures can be set up that allow access at all privilege levels (0 through 3). More privileged call gates (with DPLs of 0 or 1) can then be set up for other operating system services that are intended to be used only by the operating system (such as procedures that initialize device drivers). 4.8.5. Stack Switching Whenever a call gate is used to transfer program control to a more privileged nonconforming code segment (that is, when the DPL of the nonconforming destination code segment is less than the CPL), the processor automatically switches to the stack for the destination code segment’s privilege level. This stack switching is carried out to prevent more privileged procedures from crashing due to insufficient stack space. It also prevents less privileged procedures from interfering (by accident or intent) with more privileged procedures through a shared stack. Each task must define up to 4 stacks: one for applications code (running at privilege level 3) and one for each of the privilege levels 2, 1, and 0 that are used. (If only two privilege levels are used [3 and 0], then only two stacks must be defined.) Each of these stacks is located in a separate segment and is identified with a segment selector and an offset into the stack segment (a stack pointer). The segment selector and stack pointer for the privilege level 3 stack is located in the SS and ESP registers, respectively, when privilege-level-3 code is being executed and is automatically stored on the called procedure’s stack when a stack switch occurs. Pointers to the privilege level 0, 1, and 2 stacks are stored in the TSS for the currently running task (refer to Figure 6-2 in Chapter 6, Task Management). Each of these pointers consists of a segment selector and a stack pointer (loaded into the ESP register). These initial pointers are strictly read-only values. The processor does not change them while the task is running. They are used only to create new stacks when calls are made to more privileged levels (numerically lower privilege levels). These stacks are disposed of when a return is made from the called procedure. The next time the procedure is called, a new stack is created using the initial stack pointer. (The TSS does not specify a stack for privilege level 3 because the processor does not allow a transfer of program control from a procedure running at a CPL of 0, 1, or 2 to a procedure running at a CPL of 3, except on a return.) The operating system is responsible for creating stacks and stack-segment descriptors for all the privilege levels to be used and for loading initial pointers for these stacks into the TSS. Each stack must be read/write accessible (as specified in the type field of its segment descriptor) and must contain enough space (as specified in the limit field) to hold the following items: • • • The contents of the SS, ESP, CS, and EIP registers for the calling procedure. The parameters and temporary variables required by the called procedure. The EFLAGS register and error code, when implicit calls are made to an exception or interrupt handler. 4-21 PROTECTION The stack will need to require enough space to contain many frames of these items, because procedures often call other procedures, and an operating system may support nesting of multiple interrupts. Each stack should be large enough to allow for the worst case nesting scenario at its privilege level. (If the operating system does not use the processor’s multitasking mechanism, it still must create at least one TSS for this stack-related purpose.) When a procedure call through a call gate results in a change in privilege level, the processor performs the following steps to switch stacks and begin execution of the called procedure at a new privilege level: 1. Uses the DPL of the destination code segment (the new CPL) to select a pointer to the new stack (segment selector and stack pointer) from the TSS. 2. Reads the segment selector and stack pointer for the stack to be switched to from the current TSS. Any limit violations detected while reading the stack-segmen...
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