5-4 Vol. 3

PROTECTION

Many different styles of protection schemes can be implemented with these fields

and flags. When the operating system creates a descriptor, it places values in these

fields and flags in keeping with the particular protection style chosen for an operating

system or executive. Application program do not generally access or modify these

fields and flags.

Figure 5-1. Descriptor Fields Used for Protection
Base 23:16
31 2423 22 212019 1615 13
14 12 11 87 0
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 11 870
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