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3-6 Vol. 3
PROTECTED-MODE MEMORY MANAGEMENT
Access checks can be used to protect not only against referencing an address outside
the limit of a segment, but also against performing disallowed operations in certain
segments. For example, since code segments are designated as read-only segments,
hardware can be used to prevent writes into code segments. The access rights infor
-
mation created for segments can also be used to set up protection rings or levels.
Protection levels can be used to protect operating-system procedures from unautho-
rized access by application programs.
3.2.4 Segmentation in IA-32e Mode
In IA-32e mode of Intel 64 architecture, the effects of segmentation depend on
whether the processor is running in compatibility mode or 64-bit mode. In compati
-
bility mode, segmentation functions just as it does using legacy 16-bit or 32-bit
protected mode semantics.
Figure 3-4. Multi-Segment Model
Linear Address Space
(or Physical Memory)
Segment
Registers
CS
Segment
Descriptors
LimitAccess
Base Address
SS
LimitAccess
Base Address
DS
LimitAccess
Base Address
ES
LimitAccess
Base Address
FS
LimitAccess
Base Address
GS
LimitAccess
Base Address
LimitAccess
Base Address
LimitAccess
Base Address
LimitAccess
Base Address
LimitAccess
Base Address
Stack
Code
Data
Data
Data
Data