Descriptor

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 : 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.