Mastering Assembly Programming

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 ...