Intel architecture ia-32 Manuale Utente
Vol. 3A 4-3
PROTECTION
•
Read/write (R/W) flag — (Bit 1 of a page-directory or page-table entry.) Determines the
type of access allowed to a page: read only or read-write.
type of access allowed to a page: read only or read-write.
Figure 4-1 shows the location of the various fields and flags in the data, code, and system-
segment descriptors; Figure 3-6 shows the location of the RPL (or CPL) field in a segment
selector (or the CS register); and Figure 3-14 shows the location of the U/S and R/W flags in the
page-directory and page-table entries.
segment descriptors; Figure 3-6 shows the location of the RPL (or CPL) field in a segment
selector (or the CS register); and Figure 3-14 shows the location of the U/S and R/W flags in the
page-directory and page-table entries.
Figure 4-1. Descriptor Fields Used for Protection
Base 23:16
31
24 23 22 21 20 19
16 15
13
14
12 11
8 7
0
P
Base 31:24
G
D
P
P
L
Type
1
0
4
31
16 15
0
Base Address 15:00
Segment Limit 15:00
0
Base 23:16
A
V
V
L
Limit
19:16
B
A
W
E
0
Data-Segment Descriptor
31
24 23 22 21 20 19
16 15
13
14
12 11
8 7
0
P
Base 31:24
G
D
P
P
L
Type
1
0
4
31
16 15
0
Base Address 15:00
Segment Limit 15:00
0
Base 23:16
A
V
V
L
Limit
19:16
D
A
R
C
1
Code-Segment Descriptor
31
24 23 22 21 20 19
16 15
13
14
12 11
8 7
0
P
Base 31:24
G
D
P
P
L
Type
0
4
31
16 15
0
Base Address 15:00
Segment Limit 15:00
0
Limit
19:16
System-Segment Descriptor
A
B
C
D
DPL
C
D
DPL
Accessed
Big
Conforming
Default
Descriptor Privilege Level
Conforming
Default
Descriptor Privilege Level
Reserved
E
G
R
LIMIT
W
P
G
R
LIMIT
W
P
Expansion Direction
Granularity
Readable
Segment Limit
Writable
Present
Granularity
Readable
Segment Limit
Writable
Present
0
AVL Available to Sys. Programmer’s