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.

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.

Figure 4-1. Descriptor Fields Used for Protection

Base 23:16
31 2423 22 2120 19 1615 13
14 12 11 870
P
Base 31:24 GD
P
L
Type
1
04
31 1615 0
Base Address 15:00 Segment Limit 15:00 0
Base 23:16
A
V
L
Limit
19:16
BAWE0
Data-Segment Descriptor
31 2423 22 2120 19 1615 13
14 12 11 870
P
Base 31:24 GD
P
L
Type
1
04
31 1615 0
Base Address 15:00 Segment Limit 15:00 0
Base 23:16
A
V
L
Limit
19:16
DARC1
Code-Segment Descriptor
31 2423 22 2120 19 1615 13
14 12 11 87 0
P
Base 31:24 GD
P
LType
04
31 1615 0
Base Address 15:00 Segment Limit 15:00 0
Limit
19:16
System-Segment Descriptor
A
B
C
D
DPL
Accessed
Big
Conforming
Default
Descriptor Privilege Level
Reserved
E
G
R
LIMIT
W
P
Expansion Direction
Granularity
Readable
Segment Limit
Writable
Present
0
AVL Available to Sys. Programmer’s