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<h2 class="first-section-title"><a name="282"></a><a name="ch02lev1sec5"></a>Closing Thoughts</h2><p class="first-para">Looking at the previous three protected mode operating systems (MMURTL, Linux, and Windows), you should notice a trend. Segmentation-based protection is not utilized to its full potential in any of the case studies. I suspect that this is because of the way that virtual memory works on Intel machines. The Pentium's paging facilities, in addition to supporting memory expansion via disk storage, also provide a degree of segmentation functionality. In fact, not only does Intel paging support memory segmentation, but it also does so at a much finer level of granularity. Access policies can be instituted on a 4KB page level.</p>
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<p class="first-para">As I have mentioned several times before, operating system code serves as a policy maker that mandates how the hardware-based memory management tools will be used. Deciding not to use a given mechanism at all is still a policy decision in and of itself.</p>
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<p class="para">If you think about it, segmentation and paging both serve to break up a region of memory into smaller parts. Paging just partitions memory into smaller chunks. So, in a sense, investing in an elaborate segmentation scheme via GDTs and LDTs is somewhat of a wasted effort when the same type of services can be built on an existing service that is already being used for something else.</p>
<p class="last-para">The trade-off to relying heavily on paging is that it only permits a two-ring privilege model. This is a far cry from the four-ring privilege scheme that segmentation hardware supports. Page directory and page table entries have only a single bit to specify privilege. This leaves us with a pretty limited user/supervisor implementation of access privileges. Paging also requires more memory within the kernel itself because the data structures that track pages of data are more numerous. Most operating systems give each process its own page directory, which necessarily implies a handful of page tables and their entries. A pure segmentation scheme could potentially only require a single entry in the GDT to delimit and manage an application in memory.</p>
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