Protected mode - segmentation

While it was all fine and simple in real mode, things become a bit more complicated when it comes to protected mode. Unfortunately, memory segmentation is still intact, but the segment register no longer contain addresses. Instead, they are loaded with the so-called selectors, which are, in turn, the indices into the descriptor table multiplied by 8 (shifted 3 bits to the left). The two least significant bits designate the requested privilege level (0 for kernel space to 3 for user land). The third bit (at index 2) is the TI bit (table indicator), which indicates whether the descriptor being referred is in a global descriptor table (0) or in a local descriptor table (1). The memory descriptor is a tiny 8-byte structure, which describes the range of physical memory, its access rights, and some additional attributes:

Descriptors are stored in at least two tables:

• GDT: Global descriptor table (used by the operating system)
• LDT: Local descriptor table (per task descriptor table)

As we may conclude, the organization of memory in protected mode is not that different from that in real mode after all.

There are other types of descriptors--interrupt descriptors (stored in the interrupt description table (IDT)) and system descriptors; however, since these are in use in kernel space only, we will not discuss them, as that falls out of the scope of this book.