GXM datasheet(68/244 Pages) NSC

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Revision 3.1

Processor Programming (Continued)

Segment Present:

If = 1: Segment is memory segment allocated.

If = 0: The BASE and LIMIT fields become available for use by the system. Also, If = 0, a segment-

not-present exception generated when selector for the descriptor is loaded into a segment register

allowing virtual memory management.

14:13

DPL

Descriptor Privilege Level:

If = 00: Highest privilege level

If = 11: Low privilege level

Descriptor Type:

If = 1: Code or data segment

If = 0: System segment

11:8

TYPE

Segment Type: Refer to Table 3-23 on page 68 for TYPE bit definitions.

Bit 11 = Executable

Bit 10 = Conforming if bit 12 = 1

Bit 10 = Expand Down if bit 12 = 0

Bit 9 = Readable, if Bit 12 = 1

Bit 9 = Writable, if Bit 12 = 0

Bit 8 = Accessed

Table 3-22. Application and System Segment Descriptors Bit Definitions (Continued)

Bit

Memory

Offset

Name

Description

Table 3-23. Application and System Segment Descriptors TYPE Bit Definitions

TYPE

Bits [11:8]

System Segment and Gate Types

Bit 12 = 0

Application Segment Types

Bit 12 = 1

Num

SEWA

TYPE (Data Segments)

0000

Reserved

Data

Read-Only

0001

Available 16-Bit TSS

Data

Read-Only, accessed

0010

LDT

Data

Read/Write

0011

Busy 16-Bit TSS

Data

Read/Write accessed

0100

16-Bit Call Gate

Data

Read-Only, expand down

0101

Task Gate

Data

Read-Only, expand down, accessed

0110

16-Bit Interrupt Gate

Data

Read/Write, expand down

0111

16-Bit Trap Gate

Data

Read/Write, expand down, accessed

Num

SCRA

TYPE (Code Segments)

1000

Reserved

Code

Execute-Only

1001

Available 32-Bit TSS

Code

Execute-Only, accessed

1010

Reserved

Code

Execute/Read

1011

Busy 32-Bit TSS

Code

Execute/Read, accessed

1100

32-Bit Call Gate

Code

Execute/Read, conforming

1101

Reserved

Code

Execute/Read, conforming, accessed

1110

32-Bit Interrupt Gate

Code

Execute/Read-Only, conforming

1111

32-Bit Trap Gate

Code

Execute/Read-Only, conforming accessed

S = Code Segment (not Data Segment)

E = Expand Down

W = Write Enable

A = Accessed

C = Conforming Code Segment

R = Read Enable

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