Note that segments selectors d1 and d2 are identical

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Unformatted text preview: f the calling procedure). This is true, even if the RPL of the segment selector is different from the CPL. ACCESSING CONFORMING CODE SEGMENTS When accessing conforming code segments, the CPL of the calling procedure may be numerically equal to or greater than (less privileged) the DPL of the destination code segment; the processor generates a general-protection exception (#GP) only if the CPL is less than the DPL. (The segment selector RPL for the destination code segment is not checked if the segment is a conforming code segment.) In the example in Figure 4-6, code segment D is a conforming code segment. Therefore, calling procedures in both code segment A and B can access code segment D (using either segment selector D1 or D2, respectively), because they both have CPLs that are greater than or equal to the DPL of the conforming code segment. For conforming code segments, the DPL represents the numerically lowest privilege level that a calling procedure may be at to successfully make a call to the code segment. (Note that segments selectors D1 and D2 are identical except for their respective RPLs. But since RPLs are not checked when accessing conforming code segments, the two segment selectors are essentially interchangeable.) When program control is transferred to a conforming code segment, the CPL does not change, even if the DPL of the destination code segment is less than the CPL. This situation is the only one where the CPL may be different from the DPL of the current code segment. Also, since the CPL does not change, no stack switch occurs. Conforming segments are used for code modules such as math libraries and exception handlers, which support applications but do not require access to protected system facilities. These modules are part of the operating system or executive software, but they can be executed at numerically higher privilege levels (less privileged levels). Keeping the CPL at the level of a calling code segment when switching to a conforming code segment prevents an application program from accessing nonconforming code segments while at the privilege level (DPL) of a conforming code segment and thus prevents it from accessing more privileged data. Most code segments are nonconforming. For these segments, program control can be transferred only to code segments at the same level of privilege, unless the transfer is carried out through a call gate, as described in the following sections. 4-15 PROTECTION 4.8.2. Gate Descriptors To provide controlled access to code segments with different privilege levels, the processor provides special set of descriptors called gate descriptors. There are four kinds of gate descriptors: • • • • Call gates Trap gates Interrupt gates Task gates Task gates are used for task switching and are discussed in Chapter 6, Task Management. Trap and interrupt gates are special kinds of call gates used for calling exception and interrupt handlers. The are described in Chapter 5, Interrupt and Exception Handling. This chapter is concerned only with call gates. 4.8.3. Call Gates Call gates facilitate controlled transfers of program control between different privilege levels. They are typically used only in operating systems or executives that use the privilege-level protection mechanism. Call gates are also useful for transferring program control between 16-bit and 32-bit code segments, as described in Section 17.4., “Transferring Control Among MixedSize Code Segments” in Chapter 17, Mixing 16-Bit and 32-Bit Code. Figure 4-7 shows the format of a call-gate descriptor. A call-gate descriptor may reside in the GDT or in an LDT, but not in the interrupt descriptor table (IDT). It performs six functions: • • • • • • It specifies the code segment to be accessed. It defines an entry point for a procedure in the specified code segment. It specifies the privilege level required for a caller trying to access the procedure. If a stack switch occurs, it specifies the number of optional parameters to be copied between stacks. It defines the size of values to be pushed onto the target stack: 16-bit gates force 16-bit pushes and 32-bit gates force 32-bit pushes. It specifies whether the call-gate descriptor is valid. 4-16 PROTECTION 31 16 15 14 13 12 11 87 6 000 54 0 Offset in Segment 31:16 31 P D P L Type 01100 Param. Count 0 4 16 15 Segment Selector Offset in Segment 15:00 0 DPL Descriptor Privilege Level P Gate Valid Figure 4-7. Call-Gate Descriptor The segment selector field in a call gate specifies the code segment to be accessed. The offset field specifies the entry point in the code segment. This entry point is generally to the first instruction of a specific procedure. The DPL field indicates the privilege level of the call gate, which in turn is the privilege level required to access the selected procedure through the gate. The P flag indicates whether the call-gate descriptor is valid. (The presence of the code segment to which the gate points is indicated by the P flag in the code segment’s...
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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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