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INTEL386

Intel386 SX MICROPROCESSOR

Intel 

Intel386TM SX MICROPROCESSOR

(Present) Bit is 1 if the segment is loaded in physical

memory If Pe0 then any attempt to access this

segment causes a not present exception (exception

11) The Descriptor Privilege Level DPL is a two bit

field which specifies the protection level 0–3 asso-

ciated with a segment

The Intel386 SX Microprocessor has two main cate-

gories of segments system segments and non-sys-

tem segments (for code and data) The segment bit

S determines if a given segment is a system seg-

ment or a code or data segment If the S bit is 1 then

the segment is either a code or data segment if it is

0 then the segment is a system segment

Code and Data Descriptors (Se1)

Figure 4 5 shows the general format of a code and

data descriptor and Table 4 1 illustrates how the bits

in the Access Right Byte are interpreted

31

SEGMENT BASE 15 0

BASE 31

24

G

D

0

AVL

LIMIT

19 16

D B 1eDefault Instructions Attributes are 32-Bits

0eDefault Instruction Attributes are 16-Bits

AVL Available field for user or OS

0

SEGMENT LIMIT 15 0

0

ACCESS

RIGHTS

BYTE

BASE

23 16 a4

G Granularity Bit 1eSegment length is page granular

0eSegment length is byte granular

0

Bit must be zero (0) for compatibility with future processors

Figure 4 5 Code and Data Descriptors

Bit

Position

7

6–5

4

3

2

1

3

2

1

0

Table 4 1 Access Rights Byte Definition for Code and Data Descriptors

Name

Function

Present (P)

Descriptor Privilege

Level (DPL)

Segment Descrip-

tor (S)

Executable (E)

Expansion Direc-

tion (ED)

Writeable (W)

Executable (E)

Conforming (C)

Readable (R)

Accessed (A)

Pe1

Pe0

Segment is mapped into physical memory

No mapping to physical memory exists base and limt are

not used

Segment privilege attribute used in privilege tests

S e 1 Code or Data (includes stacks) segment descriptor

S e 0 System Segment Descriptor or Gate Descriptor

E e 0 Descriptor type is data segment

ED e 0 Expand up segment offsets must be s limit

ED e 1 Expand down segment offsets must be l limit

W e 0 Data segment may not be written into

W e 1 Data segment may be written into

If

Data

Segment

* (S e 1

E e 0)

Ee1

Ce1

Re0

Re1

Descriptor type is code segment

Code segment may only be executed

when CPL t DPL and CPL

remains unchanged

Code segment may not be read

Code segment may be read

If

Code

Segment

(S e 1

* E e 1)

A e 0 Segment has not been accessed

A e 1 Segment selector has been loaded into segment register

or used by selector test instructions

27

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