Chapter 3/ Hardware Briefing

value into GDTR, and the SGDT reads (stores) the value in GDTR. The LGDT instruc¬

tion is "privileged" and can only be executed by the operating system (we'll

discuss privileged instructions later on in more detail).

So far, I've been a bit vague about how the segment selector "refers" to the

segment descriptor. Now that the general process of logical-to-linear address

resolution has been spelled out, I'll take the time to be more specific.

The segment selector is a 16-bit value broken up into three fields (see Figure

3.11). The highest 13 bits (bits 15 through 3) are an index into the GDT, such

that a GDT can store at most 8,192 segment descriptors [0 (2'�� — 1)]. The

bottom two bits define the request privilege level (RPL) of the selector. There

are four possible binary values (00, 01, 10, and 11), where 0 has the high¬

est level of privilege and 3 has the lowest. We will see how RPL is used to

implement memory protection shortly.

16-bit Segment Selector

Bit 15

Bit 3

Bit 2

Bits 1,0

13-bit index into GDT, S, 192 possible entries

1 = specifies a descriptor in an LDT

t

i

0 = specifics a descriptor in a GDT

Requested privilege level (RPL)

(00 = most privilege, 11 = least privilege

Figure 3,11

Now let's take a close look at the anatomy of a segment descriptor to see just

what sort of information it stores. As you can see from Figure 3.12, there are

a bunch of fields in this 64-bit structure. For what we'll be doing in this book,

there are four elements of particular interest: the base address field (which

we've met already), type field, S flag, and DPL field.

The descriptor privilege level (DPL) defines the privilege level of the seg¬

ment being referenced. As with the RPL, the values range from 0 to 3, with 0

representing the highest degree of privilege. Privilege level is often described

in terms of three concentric rings that define four zones of privilege (Ring

0, Ring 1, Ring 2, and Ring 3). A segment with a DPL of 0 is referred to as

existing inside of Ring 0. Typically, the operating system kernel will execute

in Ring 0, the innermost ring, and user applications will execute in Ring 3,

the outermost ring.