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Vol. 3 2-5
SYSTEM ARCHITECTURE OVERVIEW
2.1.1 Global and Local Descriptor Tables
When operating in protected mode, all memory accesses pass through either the
global descriptor table (GDT) or an optional local descriptor table (LDT) as shown in
Figure 2-1. These tables contain entries called segment descriptors. Segment
descriptors provide the base address of segments well as access rights, type, and
usage information.
Each segment descriptor has an associated segment selector. A segment selector
provides the software that uses it with an index into the GDT or LDT (the offset of its
associated segment descriptor), a global/local flag (determines whether the selector
points to the GDT or the LDT), and access rights information.
To access a byte in a segment, a segment selector and an offset must be supplied.
The segment selector provides access to the segment descriptor for the segment (in
the GDT or LDT). From the segment descriptor, the processor obtains the base
address of the segment in the linear address space. The offset then provides the
location of the byte relative to the base address. This mechanism can be used to
access any valid code, data, or stack segment, provided the segment is accessible
from the current privilege level (CPL) at which the processor is operating. The CPL is
defined as the protection level of the currently executing code segment.
See Figure 2-1. The solid arrows in the figure indicate a linear address, dashed lines
indicate a segment selector, and the dotted arrows indicate a physical address. For
simplicity, many of the segment selectors are shown as direct pointers to a segment.
However, the actual path from a segment selector to its associated segment is always
through a GDT or LDT.
The linear address of the base of the GDT is contained in the GDT register (GDTR);
the linear address of the LDT is contained in the LDT register (LDTR).
2.1.1.1 Global and Local Descriptor Tables in IA-32e Mode
GDTR and LDTR registers are expanded to 64-bits wide in both IA-32e sub-modes
(64-bit mode and compatibility mode). For more information: see
Section 3.5.2,
“Segment Descriptor Tables in IA-32e Mode.”
Global and local descriptor tables are expanded in 64-bit mode to support 64-bit base
addresses, (16-byte LDT descriptors hold a 64-bit base address and various
attributes). In compatibility mode, descriptors are not expanded.
2.1.2 System Segments, Segment Descriptors, and Gates
Besides code, data, and stack segments that make up the execution environment of
a program or procedure, the architecture defines two system segments: the task-
state segment (TSS) and the LDT. The GDT is not considered a segment because it is
not accessed by means of a segment selector and segment descriptor. TSSs and LDTs
have segment descriptors defined for them.