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3-10 Vol. 3
PROTECTED-MODE MEMORY MANAGEMENT
TI (table indicator) flag
(Bit 2) — Specifies the descriptor table to use: clearing this flag
selects the GDT; setting this flag selects the current LDT.
Requested Privilege Level (RPL)
(Bits 0 and 1) — Specifies the privilege level of the selector. The priv-
ilege level can range from 0 to 3, with 0 being the most privileged
level. See
Section 5.5, “Privilege Levels”, for a description of the rela-
tionship of the RPL to the CPL of the executing program (or task) and
the descriptor privilege level (DPL) of the descriptor the segment
selector points to.
The first entry of the GDT is not used by the processor. A segment selector that points
to this entry of the GDT (that is, a segment selector with an index of 0 and the TI flag
set to 0) is used as a “null segment selector.” The processor does not generate an
exception when a segment register (other than the CS or SS registers) is loaded with
a null selector. It does, however, generate an exception when a segment register
holding a null selector is used to access memory. A null selector can be used to
initialize unused segment registers. Loading the CS or SS register with a null
segment selector causes a general-protection exception (#GP) to be generated.
Segment selectors are visible to application programs as part of a pointer variable,
but the values of selectors are usually assigned or modified by link editors or linking
loaders, not application programs.
3.4.3 Segment Registers
To reduce address translation time and coding complexity, the processor provides
registers for holding up to 6 segment selectors (see
Figure 3-7). Each of these
segment registers support a specific kind of memory reference (code, stack, or
data). For virtually any kind of program execution to take place, at least the code-
segment (CS), data-segment (DS), and stack-segment (SS) registers must be
loaded with valid segment selectors. The processor also provides three additional
data-segment registers (ES, FS, and GS), which can be used to make additional data
segments available to the currently executing program (or task).
Figure 3-6. Segment Selector
15
3
2
1
0
T
I
Index
Table Indicator
0 = GDT
1 = LDT
Requested Privilege Level (RPL)
RPL