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5.3 Combining Segment and Page Translation

Figure 5-12 combines Figure 5-2 and Figure 5-9 to summarize both phases of the transformation from a logical address to a physical address when paging is enabled. By appropriate choice of options and parameters to both phases, memory-management software can implement several different styles of memory management.

5.3.1 "Flat" Architecture

When the 80386 is used to execute software designed for architectures that don't have segments, it may be expedient to effectively "turn off" the segmentation features of the 80386. The 80386 does not have a mode that disables segmentation, but the same effect can be achieved by initially loading the segment registers with selectors for descriptors that encompass the entire 32-bit linear address space. Once loaded, the segment registers don't need to be changed. The 32-bit offsets used by 80386 instructions are adequate to address the entire linear-address space.

5.3.2 Segments Spanning Several Pages

The architecture of the 80386 permits segments to be larger or smaller than the size of a page (4 Kilobytes). For example, suppose a segment is used to address and protect a large data structure that spans 132 Kilobytes. In a software system that supports paged virtual memory, it is not necessary for the entire structure to be in physical memory at once. The structure is divided into 33 pages, any number of which may not be present. The applications programmer does not need to be aware that the virtual memory subsystem is paging the structure in this manner.

5.3.3 Pages Spanning Several Segments

On the other hand, segments may be smaller than the size of a page. For example, consider a small data structure such as a semaphore. Because of the protection and sharing provided by segments (refer to Chapter 6 ) , it may be useful to create a separate segment for each semaphore. But, because a system may need many semaphores, it is not efficient to allocate a page for each. Therefore, it may be useful to cluster many related segments within a page.

5.3.4 Non-Aligned Page and Segment Boundaries

The architecture of the 80386 does not enforce any correspondence between the boundaries of pages and segments. It is perfectly permissible for a page to contain the end of one segment and the beginning of another. Likewise, a segment may contain the end of one page and the beginning of another.

5.3.5 Aligned Page and Segment Boundaries

Memory-management software may be simpler, however, if it enforces some correspondence between page and segment boundaries. For example, if segments are allocated only in units of one page, the logic for segment and page allocation can be combined. There is no need for logic to account for partially used pages.

5.3.6 Page-Table per Segment

An approach to space management that provides even further simplification of space-management software is to maintain a one-to-one correspondence between segment descriptors and page-directory entries, as Figure 5-13 illustrates. Each descriptor has a base address in which the low-order 22 bits are zero; in other words, the base address is mapped by the first entry of a page table. A segment may have any limit from 1 to 4 megabytes. Depending on the limit, the segment is contained in from 1 to 1K page frames. A task is thus limited to 1K segments (a sufficient number for many applications), each containing up to 4 Mbytes. The descriptor, the corresponding page-directory entry, and the corresponding page table can be allocated and deallocated simultaneously.

up: Chapter 5 -- Memory Management
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next: Chapter 6 -- Protection