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80386
4.3
SEGMENTATION
4.3.1
Segmentation
Introduction
Segmentation is one method of memory manage-
ment. Segmentation provides the basis for protec-
tion.
Segments are used to encapsulate regions of
memory which have common attributes. For exam-
ple, all of the code of a given program could
be
con-
tained
in
a segment, or an operating system table
may reside
in
a segment. All information about a
segment
is
stored
in
an
8 byte data structure called
a descriptor. All of the descriptors
in
a system are
contained
in
tables recognized by hardware.
4.3.2 Terminology
The following terms are used throughout the discus-
sion of descriptors, privilege
levels and protection:
PL: Privilege
Level-One
of the four hierarchical
privilege levels. Level 0 is the most privileged level
and level 3 is the least privileged. More privileged
levels are numerically smaller than less privileged
levels.
RPL: Requestor Privilege
Level-The
privilege level
of the original supplier of the selector. RPL is deter-
mined
by
the
least
two
significant bits of a selector.
DPL: Descriptor Privilege
Level-This
is
the least
privileged level at which a task may access that de-
scriptor (and the segment associated with that de-
scriptor). Descriptor Privilege
Level is determined by
bits 6:5
in
the Access Right Byte of a descriptor.
CPL: Current Privilege
Level-The
privilege level at
which a task
is
currently executing, which equals the
privilege
level of the code segment being executed.
15
LDTR
I
LOT
DESCR
SELECTOR
15
I
lOT
LIMIT
I
lOT
BASE
IDTR
LINEAR
ADDRESS
31
15
I
GOT
LIMIT
I
GOT
BASE
GDTR
LINEAR
ADDRESS
31
0
0
0
0
0
CPL can also be determined by examining the low-
est 2 bits of the
CS
register, except for conforming
code segments.
EPL: Effective Privilege
Level-The
effective privi-
lege level
is
the least privileged of the RPL and
DPL.
Since smaller privilege level values indicate greater
privilege, EPL
is
the numerical maximum of RPL and
DPL.
Task: One instance of the execution of a program.
Tasks are
also referred to as processes.
4.3.3 Descriptor Tables
4.3.3.1 DESCRIPTOR TABLES INTRODUCTION
The descriptor tables define all of the segments
which are used
in
an 80386 system. There are three
types of
tables on the 80386 which hold descriptors:
the
Global Descriptor Table, Local Descriptor Table,
and the Interrupt Descriptor Table. All of the tables
are variable length memory arrays. They can range
in
size between 8 bytes and 64K bytes. Each table
can hold
up
to 8192 8 byte descriptors. The upper
13 bits of a
selector are used
as
an index into the
descriptor
table. The tables have registers associat-
ed
with them which hold the 32-bit linear base ad-
dress, and the 16-bit
limit of each table.
Each of the tables has a register associated with it
the GDTR, LDTR, and the
IDTR
(see Figure 4-3).
The LGDT, LLDT, and LlDT instructions,
load the
base and
limit of the Global, Local, and Interrupt De-
scriptor
Tables, respectively, into the appropriate
register. The
SGDT,
SLOT,
and SlOT store the base
and
limit values. These tables
are
manipulated by
the operating system. Therefore, the
load descriptor
table instructions are privileged instructions.
--------------
15
0
I
LOT
LIMIT
l
LOT
BASE
LINEAR
ADDRESS
32
PROGRAM
INVISIBLE
AUTOMATICALLY
LOADED
FROM
LOT
DESCRIPTOR
--------------
231630-57
Figure 4·3.
Descriptor
Table Registers
35