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2-16 Vol. 3
SYSTEM ARCHITECTURE OVERVIEW
2.4.1 Global Descriptor Table Register (GDTR)
The GDTR register holds the base address (32 bits in protected mode; 64 bits in
IA-32e mode) and the 16-bit table limit for the GDT. The base address specifies the
linear address of byte 0 of the GDT; the table limit specifies the number of bytes in
the table.
The LGDT and SGDT instructions load and store the GDTR register, respectively. On
power up or reset of the processor, the base address is set to the default value of 0
and the limit is set to 0FFFFH. A new base address must be loaded into the GDTR as
part of the processor initialization process for protected-mode operation.
See also: Section 3.5.1, “Segment Descriptor Tables.”
2.4.2 Local Descriptor Table Register (LDTR)
The LDTR register holds the 16-bit segment selector, base address (32 bits in
protected mode; 64 bits in IA-32e mode), segment limit, and descriptor attributes
for the LDT. The base address specifies the linear address of byte 0 of the LDT
segment; the segment limit specifies the number of bytes in the segment. See also:
Section 3.5.1, “Segment Descriptor Tables.”
The LLDT and SLDT instructions load and store the segment selector part of the LDTR
register, respectively. The segment that contains the LDT must have a segment
descriptor in the GDT. When the LLDT instruction loads a segment selector in the
LDTR: the base address, limit, and descriptor attributes from the LDT descriptor are
automatically loaded in the LDTR.
When a task switch occurs, the LDTR is automatically loaded with the segment
selector and descriptor for the LDT for the new task. The contents of the LDTR are not
automatically saved prior to writing the new LDT information into the register.
On power up or reset of the processor, the segment selector and base address are set
to the default value of 0 and the limit is set to 0FFFFH.
Figure 2-5. Memory Management Registers
0
47(79)
GDTR
IDTR
System Table Registers
32(64)-bit Linear Base Address
16-Bit Table Limit
1516
32(64)-bit Linear Base Address
0
Tas k
LDTR
System Segment
Seg. Sel.
15
Seg. Sel.
Segment Descriptor Registers (Automatically Loaded)
32(64)-bit Linear Base Address
Segment Limit
Attributes
Registers
32(64)-bit Linear Base Address
Segment Limit
Register
16-Bit Table Limit