Intel 80286, 80287 manual Dpl, ~~~/do I I LJ. ITI~~

PROTECTION

BITSNAME

1-0 REQUESTED PRIVILEGE LEVEL (RPL)

2TABLE INDICATOR (TI)

15-3 INDEX

SELECTOR

I~~~/do I I LJ. ITI~~

8 7 2 1 0

FUNCTION

INDICATES SELECTOR PRIVILEGE

LEVEL DESIRED

TI ~ 0 USE GLOBAL DESCRIPTOR TABLE (GOT)

TI ~ 1 USE LOCAL DESCRIPTOR TABLE (LOT)

SELECT DESCRIPTOR ENTRY IN TABLE

G30108

Figure 7-7. Selector Fields

Table 7-1. Segment Access Rights Byte Format

Bit Name Description

7Present1 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.

4Segment1 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