Descriptor

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Descritors are describing System Segments, Memory Segments and Gates.

Contents

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 :
0b0101=0x5 Task gate
0b0110=0x6 16-bit interrupt gate
0b0111=0x7 16-bit trap gate
0b1110=0xE 32-bit interrupt gate
0b1111=0xF 32-bit trap gate
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 :
0b0100=0x4 16-bit Call gate
0b1100=0xc 32-bit Call gate
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.
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