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80386
attributes include the 32-bit base linear address of
the segment, the 20-bit
length and granularity of the
segment, the protection
level, read, write or execute
privileges, the default size of the operands (16-bit
or
32-bit), and the type of segment. All of the attribute
information about a segment
is
contained
in
12
bits
in
the segment descriptor. Figure 4-5 shows the gen-
eral format of a descriptor. All segments
on
the
80386 have three attribute
fields in common: the P
bit, the DPL bit, and the
S bit. The Present P bit
is
1
if the segment
is
loaded
in
physical memory, if P = 0
then any attempt to access this segment causes a
not present exception (exception 11). The Descrip-
tor
Privilege Level DPL
is
a two-bit field which speci-
fies the protection
level
0-3
associated with a seg-
ment.
The 80386 has two main categories of segments
system segments and non-system segments (for
code and data). The segment
S bit
in
the segment
descriptor determines if a given segment
is
a system
segment or a code or data segment.
If
the S bit
is
1
then the segment
is
either a code
or
data segment, if
it
is
0 then the segment
is
a system segment.
4.3.4.2386
CODE, DATA DESCRIPTORS
(S=
1)
Figure 4-6 shows the
general format of a code and
data descriptor and
Table
4-1
illustrates how the bits
in
the Access Rights Byte are interpreted.
31
o
SEGMENT BASE 15
...
0
SEGMENT LIMIT
15
...
0
o
LIMIT
ACCESS
BASE
BASE
31
...
24
G 0 0 0
19
...
16
RIGHTS
23
...
16
BYTE
+4
DIB 1
~
Default Instructions Attnbutes are 32·81ts
o
~
Default Instruction Attributes are 16·8its
G Granulanty Bit 1 - Segment length IS page granular
o
~
Segment length
is
byte granular
Type
Field
Definition
Bit must be zero
(0)
for compatibility with future processors
Figure 4-6. Segment
Descriptors
Table 4-1.
Access
Rights
Byte
Definition
for
Code
and Data Descriptions
Bit
Name
Function
Position
7
Present
(P)
P = 1
Segment
is
mapped into physical memory.
P = 0
No
mapping to physical memory exits, base and limit are
not used.
6-5
Descriptor Privilege
Segment privilege attribute used
in
privilege tests.
Level (DPL)
4 Segment Descrip- S = 1
Code or Data
(includes stacks) segment descriptor
tor
(S)
S=O
System Segment Descriptor or Gate Descriptor
3
Executable
(E)
E
~
0
D'''';ptm
typ'
;,
data
"gm,nt
r
2 Expansion Direc-
ED
= 0 Expand up segment, offsets must be
,,;
limit. Data
tion
(ED)
ED
= 1 Expand down segment, offsets must be > limit. Segment
1
Writeable
(W)
W = 0 Data segment may not be written into.
(S
=
1,
W = 1 Data segment may be written into. E =
0)
3
Executable
(E)
E = 1
Descriptor type
is
code segment:
r
2 Conforming
(C)
C = 1
Code segment may
only be executed Code
when
CPL
:2:
DPL and CPL Segment
remains unchanged.
(S
=
1,
1 Readable
(R)
R=O
Code segment may not be read.
E =
1)
R = 1
Code segment may be read.
0 Accessed
(A)
A=O
Segment has not been accessed.
A=1
Segment selector has been loaded into segment register
or used by
selector test instructions.
37