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The segment selector from this pointer identifies the

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Unformatted text preview: descriptor.) The parameter count field indicates the number of parameters to copy from the calling procedures stack to the new stack if a stack switch occurs (refer to Section 4.8.5., “Stack Switching”). The parameter count specifies the number of words for 16-bit call gates and doublewords for 32-bit call gates. Note that the P flag in a gate descriptor is normally always set to 1. If it is set to 0, a not present (#NP) exception is generated when a program attempts to access the descriptor. The operating system can use the P flag for special purposes. For example, it could be used to track the number of times the gate is used. Here, the P flag is initially set to 0 causing a trap to the not-present exception handler. The exception handler then increments a counter and sets the P flag to 1, so that on returning from the handler, the gate descriptor will be valid. 4.8.4. Accessing a Code Segment Through a Call Gate To access a call gate, a far pointer to the gate is provided as a target operand in a CALL or JMP instruction. The segment selector from this pointer identifies the call gate (refer to Figure 4-8); the offset from the pointer is required, but not used or checked by the processor. (The offset can be set to any value.) When the processor has accessed the call gate, it uses the segment selector from the call gate to locate the segment descriptor for the destination code segment. (This segment descriptor can be in the GDT or the LDT.) It then combines the base address from the code-segment descriptor with the offset from the call gate to form the linear address of the procedure entry point in the code segment. As shown in Figure 4-9, four different privilege levels are used to check the validity of a program control transfer through a call gate: 4-17 PROTECTION • • • • The CPL (current privilege level). The RPL (requestor's privilege level) of the call gate’s selector. The DPL (descriptor privilege level) of the call gate descriptor. The DPL of the segment descriptor of the destination code segment. The C flag (conforming) in the segment descriptor for the destination code segment is also checked. Far Pointer to Call Gate Segment Selector Offset Required but not used by processor Descriptor Table Offset Segment Selector Offset Call-Gate Descriptor Base Base + Procedure Entry Point Base Code-Segment Descriptor Figure 4-8. Call-Gate Mechanism 4-18 PROTECTION CS Register CPL Call-Gate Selector RPL Call Gate (Descriptor) DPL Privilege Check Destination CodeSegment Descriptor DPL Figure 4-9. Privilege Check for Control Transfer with Call Gate The privilege checking rules are different depending on whether the control transfer was initiated with a CALL or a JMP instruction, as shown in Table 4-1. Table 4-1. Privilege Check Rules for Call Gates Instruction CALL Privilege Check Rules CPL ≤ call gate DPL; RPL ≤ call gate DPL Destination conforming code segment DPL ≤ CPL Destination nonconforming code segment DPL ≤ CPL JMP CPL ≤ call gate DPL; RPL ≤ call gate DPL Destination conforming code segment DPL ≤ CPL Destination nonconforming code segment DPL = CPL The DPL field of the call-gate descriptor specifies the numerically highest privilege level from which a calling procedure can access the call gate; that is, to access a call gate, the CPL of a calling procedure must be equal to or less than the DPL of the call gate. For example, in Figure 4-12, call gate A has a DPL of 3. So calling procedures at all CPLs (0 through 3) can access this call gate, which includes calling procedures in code segments A, B, and C. Call gate B has a DPL of 2, so only calling procedures at a CPL or 0, 1, or 2 can access call gate B, which includes calling procedures in code segments B and C. The dotted line shows that a calling procedure in code segment A cannot access call gate B. 4-19 PROTECTION The RPL of the segment selector to a call gate must satisfy the same test as the CPL of the calling procedure; that is, the RPL must be less than or equal to the DPL of the call gate. In the example in Figure 4-12, a calling procedure in code segment C can access call gate B using gate selector B2 or B1, but it could not use gate selector B3 to access call gate B. If the privilege checks between the calling procedure and call gate are successful, the processor then checks the DPL of the code-segment descriptor against the CPL of the calling procedure. Here, the privilege check rules vary between CALL and JMP instructions. Only CALL instructions can use call gates to transfer program control to more privileged (numerically lower privilege level) nonconforming code segments; that is, to nonconforming code segments with a DPL less than the CPL. A JMP instruction can use a call gate only to transfer program control to a nonconforming code segment with a DPL equal to the CPL. CALL and JMP instruction can both transfer program control to a more privileged conforming code segment; that is, to a conforming code segment with a DPL less than or equal to the CPL. If a call is made to a more privileged (numerically lower privilege level) nonconforming destination code segment, the CPL is...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at Berkeley.

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