User's Manual

PROTECTION
SELECTOR
I~~~/ do I I LJ. I T I ~~
8 7 2 1 0
BITS
NAME
FUNCTION
1-0
REQUESTED
INDICATES SELECTOR PRIVILEGE
PRIVILEGE
LEVEL DESIRED
LEVEL (RPL)
2
TABLE
TI ~ 0 USE GLOBAL DESCRIPTOR TABLE
INDICATOR
(GOT)
(TI)
TI ~ 1 USE LOCAL DESCRIPTOR TABLE
(LOT)
15-3
INDEX
SELECT DESCRIPTOR ENTRY
IN TABLE
G30108
Figure 7-7. Selector Fields
Table 7-1. Segment Access Rights Byte Format
Bit Name
Description
7 Present 1
means Present and addressable in real memory; 0 means not
present.
See section 11.3.
6,5
DPL 2-bit Descriptor Privilege Level, 0 to 3.
4 Segment
1 means Segment descriptor; 0 means control descriptor.
For
Segment= 1, the remaining bits have the following meanings:
3
Executable
1 means code,
0 means data.
2
C or ED If code, Conforming: 1 means yes, 0 no.
If data, Expand Down: 1 yes, 0 no-normal case.
1 RorW
If code, Readable: 1 means readable, 0 not.
If data, Writable: 1 means writable, 0 not.
0
Accessed
1 if segment descriptor has been Accessed,
0 if not.
NOTE: When the Segment bit (bit 4) is 0, the descriptor is for a gate, a task state segment, or a Local
Descriptor Table, and the meanings of bits 0 through 3 change. Control transfers and descriptors
are discussed
in section 7.5.
For example, the access rights byte for a data and code segment present in real memory but not yet
accessed (at the same privilege level)
is shown in figure 7-8.
Whenever a segment descriptor is loaded into a segment register, the accessed bit in the descriptor
table
is set to I. This bit is useful for determining the usage profile of the segment.
7-11