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回复 6# tianhailong
那就不清楚了，请看《understanding the linux kernel the 3rd. edition》的第二章内存寻址里面的Linux LDT一节里面讲的内容：
Most Linux User Mode applications do not make use of a Local Descriptor Table, thus the kernel defines a default LDT to be shared by most processes. The default Local Descriptor Table is stored in the default_ldt array. It includes five entries, but only two of them are effectively used by the kernel: a call gate for iBCS executables, and a call gate for Solaris /x86 executables (see the section "Execution Domains" in Chapter 20). Call gates are a mechanism provided by 80 x 86 microprocessors to change the privilege level of the CPU while invoking a predefined function; as we won't discuss them further, you should consult the Intel documentation for more details.
In some cases, however, processes may require to set up their own LDT. This turns out to be useful to applications (such as Wine) that execute segment-oriented Microsoft Windows applications. The modify_ldt( ) system call allows a process to do this.
Any custom LDT created by modify_ldt( ) also requires its own segment. When a processor starts executing a process having a custom LDT, the LDT entry in the CPU-specific copy of the GDT is changed accordingly.
User Mode applications also may allocate new segments by means of modify_ldt( ); the kernel, however, never makes use of these segments, and it does not have to keep track of the corresponding Segment Descriptors, because they are included in the custom LDT of the process. |
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发表于 2010-04-20 11:15
