The LDT_ENTRY structure describes an entry in the descriptor table. This structure is valid only on x86-based systems.
typedef struct _LDT_ENTRY { // ldte
WORD LimitLow;
WORD BaseLow;
union {
struct {
BYTE BaseMid;
BYTE Flags1;
BYTE Flags2;
BYTE BaseHi;
} Bytes;
struct {
DWORD BaseMid : 8;
DWORD Type : 5;
DWORD Dpl : 2;
DWORD Pres : 1;
DWORD LimitHi : 4;
DWORD Sys : 1;
DWORD Reserved_0 : 1;
DWORD Default_Big : 1;
DWORD Granularity : 1;
DWORD BaseHi : 8;
} Bits;
} HighWord;
} LDT_ENTRY, *PLDT_ENTRY;
Members
LimitLow
Contains the low 16 bits of the address of the last byte in the segment.
BaseLow
Contains the low 16 bits of the base address of the segment.
HighWord
Contains the high two words of the descriptor. This member may be interpreted as bytes or collections of bits, depending on the level of detail required.
The members of the Bits structure are as follows:
| Member | Contents |
| BaseMid | Middle bits (16-23) of the base address of the segment. |
| Type | Bitmask that indicates the type of segment. This member can be one of the following values: |
| Value | Meaning |
| 0 | Read-only data segment |
| 1 | Read-write data segment |
| 2 | Unused segment |
| 3 | Read-write expand-down data segment |
| 4 | Execute-only code segment |
| 5 | Executable-readable code segment |
| 6 | Execute-only "conforming" code segment |
| 7 | Executable-readable "conforming" code segment |
| Dpl | Privilege level of the descriptor. This member is an integer value in the range 0 (most privileged) through 3 (least privileged). |
| Pres | Present flag. This member is 1 if the segment is present in physical memory or 0 if it is not. |
| LimitHi | High bits (16-19) of the address of the last byte in the segment. |
| Sys | Space that is available to system programmers. This member might be used for marking segments in some system-specific way. |
| Reserved_0 | Reserved. |
| Default_Big | Size of segment. If the segment is a data segment, this member contains 1 if the segment is larger than 64 kilobytes (K) or 0 if the segment is smaller than or equal to 64K. |
| If the segment is a code segment, this member contains 1 if the segment is a code segment and runs with the default (native mode) instruction set. This member contains 0 if the code segment is an 80286 code segment and runs with 16-bit offsets and the 80286-compatible instruction set. | |
| Granularity | Granularity. This member contains 0 if the segment is byte granular, 1 if the segment is page granular. |
| BaseHi | High bits (24-31) of the base address of the segment. |
The members of the Bytes structure are as follows:
| Member | Contents |
| BaseMid | Middle bits (16-23) of the base address of the segment. |
| Flags1 | Values of the Type, Dpl, and Pres members in the Bits structure. |
| Flags2 | Values of the LimitHi, Sys, Reserved_0, Default_Big, and Granularity members in the Bits structure. |
| BaseHi | High bits (24-31) of the base address of the segment. |
Remarks
The GetThreadSelectorEntry function fills this structure with information from an entry in the descriptor table. You can use this information to convert a segment-relative address to a linear virtual address.
The base address of a segment is the address of offset 0 in the segment. To calculate this value, combine the BaseLow, BaseMid, and BaseHi members.
The limit of a segment is the address of the last byte that can be addressed in the segment. To calculate this value, combine the LimitLow and LimitHi members.
See Also