Descriptors are describing System Segments, Memory Segments and Gates.
System Segment Descriptor
| Name
|
Bit
|
Full Name
|
Description
|
| Limit
|
0..15 |
Limit 0..15 |
Memory range.
|
| BAddr
|
16..39 |
Base Address 0..23 |
Linear address where the memory range begins.
|
| Typ
|
40..43 |
Descriptor Type |
Possible types :
| 0000b=0x0 |
unused (invalid Descriptor)
|
| 0001b=0x1 |
80286-TSS, 16 bit
|
| 0010b=0x2 |
LDT
|
| 0011b=0x3 |
activ 80286-TSS, 16 bit
|
| 1001=0x9 |
80386-TSS, 32 bit
|
| 1011=0xB |
activ 80386-TSS, 32 bit
|
|
| S
|
44 |
Storage Segment |
= 0 for System Segments.
|
| DPL
|
45,46 |
Descriptor Privilege Level |
Specifies which privilege level a process minimum must have to use this Descriptor.
|
| P
|
47 |
Present |
Set this to 1 to be able to access the segment, otherwise a #NP (Segment not Present) exception is generated on access.
|
| Limit
|
48..51 |
Limit 16..19 |
high part of the Limit.
|
| AVL
|
52 |
|
.
|
| 0
|
53 |
|
.
|
| -
|
54 |
|
.
|
| G
|
55 |
|
.
|
| BAddr
|
56..63 |
Base Address 24..31 |
Higher part of the Base Address.
|
Memory Segment Descriptor
| Name
|
Bit
|
Full Name
|
Description
|
| Limit
|
0..15 |
Limit 0..15 |
Memory range.
|
| BAddr
|
16..39 |
Base Address 0..23 |
Linear address where the memory range begins.
|
| A
|
40 |
|
|
| WR
|
41 |
|
|
| CE
|
42 |
|
|
| EX
|
43 |
|
|
| S
|
44 |
Storage Segment |
= 1 for Memory Segments.
|
| DPL
|
45,46 |
Descriptor Privilege Level |
Specifies which privilege level a process minimum must have to use this Descriptor.
|
| P
|
47 |
Present |
Set this to 1 to be able to access the segment, otherwise a #NP (Segment not Present) exception is generated on access.
|
| Limit
|
48..51 |
Limit 16..19 |
high part of the Limit.
|
| AVL
|
52 |
|
.
|
| 0
|
53 |
|
.
|
| B
|
54 |
|
.
|
| G
|
55 |
|
.
|
| BAddr
|
56..63 |
Base Address 24..31 |
Higher part of the Base Address.
|
Interrupt Gate Descriptor
| Name
|
Bit
|
Full Name
|
Description
|
| Offset
|
0..15 |
Offset 0..15 |
Lower part of the offset address (also known as pointer).
|
| Selector
|
16..31 |
Selector |
Selector that specifies the segment of the function to call.
|
| Reserved
|
32..35 |
|
|
| 0
|
36..39 |
- |
Have to be 0.
|
| Type
|
40..43 |
Gate Type |
Possible interrupt gate types :
|
| S
|
44 |
Storage Segment |
= 0 for gates.
|
| DPL
|
45,46 |
Descriptor Privilege Level |
Gate call protection. Specifies which privilege level the calling process minimum should have.
|
| P
|
47 |
Present |
Set this to 1 to be able to access the segment, otherwise a #NP (Segment not Present) exception is generated on access.
|
| Offset
|
48..63 |
Offset 16..31 |
Higher part of the offset.
|
Call Gate Descriptor
| Name
|
Bit
|
Full Name
|
Description
|
| Offset
|
0..15 |
Offset 0..15 |
Lower part of the offset address (also known as pointer).
|
| Selector
|
16..31 |
Selector |
Selector that specifies the code segment of the function to call.
|
| ParamCnt
|
32..35 |
uint32_t's to copy |
Number of uint32_t stack arguments to copy on stack switch.
|
| 0
|
36..39 |
- |
Have to be 0.
|
| Type
|
40..43 |
Gate Type |
Possible call gate types :
|
| S
|
44 |
Storage Segment |
= 0 for gates.
|
| DPL
|
45,46 |
Descriptor Privilege Level |
Gate call protection. Specifies which privilege level the calling process minimum should have.
|
| P
|
47 |
Present |
Set this to 1 to be able to access the segment, otherwise a #NP (Segment not Present) exception is generated on access.
|
| Offset
|
48..63 |
Offset 16..31 |
Higher part of the offset.
|
