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3-8 Vol. 3
PROTECTED-MODE MEMORY MANAGEMENT
3.3.1 Intel
®
64 Processors and Physical Address Space
On processors that support Intel 64 architecture (CPUID.80000001:EDX[29] = 1),
the size of the physical address range is implementation-specific and indicated by
CPUID.80000008H:EAX[bits 7-0].
For the format of information returned in EAX, see “CPUID—CPU Identification” in
Chapter 3 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual,
Volume 2A. See also: Chapter 4, “Paging.”
3.4 LOGICAL AND LINEAR ADDRESSES
At the system-architecture level in protected mode, the processor uses two stages of
address translation to arrive at a physical address: logical-address translation and
linear address space paging.
Even with the minimum use of segments, every byte in the processor’s address
space is accessed with a logical address. A logical address consists of a 16-bit
segment selector and a 32-bit offset (see Figure 3-5). The segment selector identi-
fies the segment the byte is located in and the offset specifies the location of the byte
in the segment relative to the base address of the segment.
The processor translates every logical address into a linear address. A linear address
is a 32-bit address in the processor’s linear address space. Like the physical address
space, the linear address space is a flat (unsegmented), 2
32
-byte address space,
with addresses ranging from 0 to FFFFFFFFH. The linear address space contains all
the segments and system tables defined for a system.
To translate a logical address into a linear address, the processor does the following:
1. Uses the offset in the segment selector to locate the segment descriptor for the
segment in the GDT or LDT and reads it into the processor. (This step is needed
only when a new segment selector is loaded into a segment register.)
2. Examines the segment descriptor to check the access rights and range of the
segment to insure that the segment is accessible and that the offset is within the
limits of the segment.
3. Adds the base address of the segment from the segment descriptor to the offset
to form a linear address.