GXM Datasheet, PDF (68/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

Processor Programming (Continued)
Table 3-22. Application and System Segment Descriptors Bit Definitions (Continued)
Memory
Bit Offset
15
+4
14:13
+4
12
+4
11:8
+4
Name
P
DPL
S
TYPE
Description
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.
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
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 3-23. 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
0
1
2
3
4
5
6
7
Num
8
9
A
B
C
D
E
F
SEWA
0000
Reserved
0001
Available 16-Bit TSS
0010
LDT
0011
Busy 16-Bit TSS
0100
16-Bit Call Gate
0101
Task Gate
0110
16-Bit Interrupt Gate
0111
16-Bit Trap Gate
SCRA
1000
Reserved
1001
Available 32-Bit TSS
1010
Reserved
1011
Busy 32-Bit TSS
1100
32-Bit Call Gate
1101
Reserved
1110
32-Bit Interrupt Gate
1111
32-Bit Trap Gate
S = Code Segment (not Data Segment)
E = Expand Down
W = Write Enable
TYPE (Data Segments)
Data Read-Only
Data Read-Only, accessed
Data Read/Write
Data Read/Write accessed
Data Read-Only, expand down
Data Read-Only, expand down, accessed
Data Read/Write, expand down
Data Read/Write, expand down, accessed
TYPE (Code Segments)
Code Execute-Only
Code Execute-Only, accessed
Code Execute/Read
Code Execute/Read, accessed
Code Execute/Read, conforming
Code Execute/Read, conforming, accessed
Code Execute/Read-Only, conforming
Code Execute/Read-Only, conforming accessed
A = Accessed
C = Conforming Code Segment
R = Read Enable
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