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    <p>Linux never used 286 style segmentation for separating processes, or otherwise making the virtual memory interestingly segmented, but rather used 386 style paging from the beginning. Transfer to kernel mode (syscall) used to be a simple <code>int</code> instruction which transfered execution according to the interrupt table and caused the CPU to enter the kernel mode (protection level 0). However, the CPU still had to reload segment descriptors to "learn" the new protection level and where the new segment is (although it was always the same dummy kernel mode segment which the CPU just "didn't know".).</p> <p>AMD and Intel came forward with <a href="http://www.codeguru.com/cpp/misc/misc/system/article.php/c8223/System-Call-Optimization-with-the-SYSENTER-Instruction.htm" rel="nofollow">optimized instructions</a> to make this process faster and this is what all operating systems on this platform use in reality.</p> <p>Kernel code then has to do even more work to save registers on the stack and initialize them to new values, and this has not changed. But this is normally not understood to be a part of the system call process.</p> <blockquote> <p>When system call is made, is previlege level checked using code segment registers or control register are used?</p> </blockquote> <p>The privilege level is <em>obtained</em>, not checked, from the <em>new</em> code segment as referenced through the interrupt table - or, in the optimized case, as pre-loaded into a MSR (a CPU register not accessible by non-kernel code).</p> <p>Another way of saying the same is that the switch to level 0 happens automatically on CPU level, but the segment descriptors and/or MSRs need to be prearranged by the kernel in a way that really results in kernel executing the trap handler and not just a general protection fault.</p>
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    1. COI had one more double.is kernel mode in linux in the same process address space which used system call to enter kernel mode or is kernel code is completely a different process?
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    2. CO@vindhya - Process and address space are different things. Address spaces are provided mostly by CPU. Processes are provided by the operating system; each process consists of an address space, kernel mode stack space, table of open files, and so on. Address spaces for two processes overlap completely (if they are just threads of what you would call "the same process"), or partly, with all of kernel memory always mapped to the same addresses, but not accessible from user mode. System call does not change the process, and it does not change the address space.
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