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