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Vol. 3 3-9
PROTECTED-MODE MEMORY MANAGEMENT
If paging is not used, the processor maps the linear address directly to a physical
address (that is, the linear address goes out on the processor’s address bus). If the
linear address space is paged, a second level of address translation is used to trans
-
late the linear address into a physical address.
See also: Chapter 4, “Paging.”
3.4.1 Logical Address Translation in IA-32e Mode
In IA-32e mode, an Intel 64 processor uses the steps described above to translate a
logical address to a linear address. In 64-bit mode, the offset and base address of the
segment are 64-bits instead of 32 bits. The linear address format is also 64 bits wide
and is subject to the canonical form requirement.
Each code segment descriptor provides an L bit. This bit allows a code segment to
execute 64-bit code or legacy 32-bit code by code segment.
3.4.2 Segment Selectors
A segment selector is a 16-bit identifier for a segment (see Figure 3-6). It does not
point directly to the segment, but instead points to the segment descriptor that
defines the segment. A segment selector contains the following items:
Index (Bits 3 through 15) — Selects one of 8192 descriptors in the GDT or
LDT. The processor multiplies the index value by 8 (the number of
bytes in a segment descriptor) and adds the result to the base address
of the GDT or LDT (from the GDTR or LDTR register, respectively).
Figure 3-5. Logical Address to Linear Address Translation
Offset (Effective Address)
0
Base Address
Descriptor Table
Segment
Descriptor
31(63)
Seg. Selector
015
Logical
Address
+
Linear Address
031(63)