User's Manual

3-22 Vol. 3
PROTECTED-MODE MEMORY MANAGEMENT
3.5.2 Segment Descriptor Tables in IA-32e Mode
In IA-32e mode, a segment descriptor table can contain up to 8192 (2
13
) 8-byte
descriptors. An entry in the segment descriptor table can be 8 bytes. System descrip-
tors are expanded to 16 bytes (occupying the space of two entries).
GDTR and LDTR registers are expanded to hold 64-bit base address. The corre-
sponding pseudo-descriptor is 80 bits. (see the bottom diagram in Figure 3-11).
The following system descriptors expand to 16 bytes:
— Call gate descriptors (see Section 5.8.3.1, “IA-32e Mode Call Gates”)
— IDT gate descriptors (see Section 6.14.1, “64-Bit Mode IDT”)
— LDT and TSS descriptors (see Section 7.2.3, “TSS Descriptor in 64-bit
mode”).
Figure 3-11. Pseudo-Descriptor Formats
0
32-bit Base Address
Limit
47 1516
0
64-bit Base Address
Limit
79 1516