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386 Memory Management Vitals
Most popular microprocessor use either segmentation or paging to manage memory address space access. The 386 is rare in that it possesses both. In fact, since segmentation, (Figure 3(a)), is placed on top of paging (Figure 3(b)), you are expected to use segmentation in some form any time memory is paged. And, most important, BSD relies on paging.
All operand references on the 386 are tied to one of the segment registers. This segment register uses a 16-bit selector (low-order bits determine level of access) to find a descriptor. This descriptor then determines the location of underlying memory in linear address space. When segmentation alone is enabled (also known as protected mode), the linear address space corresponds to the physical address of the selected segment for the operand. However, when paging is implemented, the linear address space address must be run through a two-level paging mechanism to find the physical page frame number, the actual address of physical memory underneath the virtual address.
One of the most powerful, yet confusing, features of the 386 is its segmented architecture. While the current trend in microprocessors has been oriented towards a single "flat" linear virtual address space, the 386 has continued the bias toward segments held by the entire 80x86 line. The two most important changes in the 386 from previous versions -- permitting 32-bit operations and expanding segments from 64 Kbytes to 4 gigabytes in size -- may turn some of the inherent disadvantages of 80x86 segments into an advantage. Segments once too small for many data items (such as arrays of real numbers) can now utilize alternative address spaces. This is of great interest to those working with specialized applications, such as 3-D to 2-D transformations.
Copyrightę1994 Willaim & Lynne Jolitz