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3.3. BASIC CONCEPTS CHAPTER 3. VM, PROTECTION AND PAGING
Figure 3.1: Protected Mode Segment Descriptor ((c) Dr. Jim Plusquellic [2])
3.3 Basic Concepts
3.3.1 Protected Mode
The IA32 Architecture offers a feature-set called "Protected Mode", which makes task
separation possible. Using the protected mode capabilities, we can place restrictions
on what a piece of code is allowed to do and in which part of the physical memory
it may work. The two mechanisms making this possible are protected-mode-
Segmentation and Paging. They will be explained in this chapter. Note that in order to
switch to Protected Mode, at the very least the special registers GDTR and IDTR must
have been loaded.
3.3.2 Protected Mode Segmentation
Protected Mode Segmentation works differently than Real Mode Segmentation. In this
document we consider only Protected Mode Segmentation.
Basically it works by defining memory segments using so-called Segment Descriptors.
They contain a Start Address in linear memory and a Limit to how much memory
beyond the start address a program is allowed to access. The segment descriptor also
defines restrictions on the kind of operations a program can execute. Otherwise a Segment
Violation is triggered. Even before switching to Protected Mode, some Segment
Descriptors need to be defined in the so-called Global Descriptor Table (short GDT).
Like any descriptor table, the GDT is an array of Segment Descriptors of 8 byte length.
Obviously, a restricted program can not be allowed to (re)write the GDT and the protection
mechanisms available should be employed to prevent this. The GDT should
contain at least one reasonable Segment Descriptor beyond the first entry. The first
entry also called the Null Descriptor, has a special purpose and all its values set to
zero. Once a GDT has been set up, we can point the CPU to its location in memory by
loading its linear address into the Global Descriptor Table Register (short GDTR) using
the LGDT instruction. Figure 3.1 shows the fields of a Segment Descriptor. Table 3.1
explains their use.
Obviously, defining just one set of Descriptors for all programs is not going to help
much with Task Separation, so conveniently the Local Descriptor Table (short LDT)
comes to the rescue. The LDT, just like the GDT, is an array of Segment Descriptors,
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