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LDT_ENTRY structure (winnt.h)

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Describes an entry in the descriptor table. This structure is valid only on x86-based systems.

Syntax

typedef struct _LDT_ENTRY {
  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

The low-order part of the address of the last byte in the segment.

BaseLow

The low-order part of the base address of the segment.

HighWord

The high-order portion of the descriptor. This member may be interpreted as bytes or collections of bits, depending on the level of detail required.

HighWord.Bytes

HighWord.Bytes.BaseMid

Middle bits (16–23) of the base address of the segment.

HighWord.Bytes.Flags1

Values of the Type, Dpl, and Pres members in the Bits structure.

HighWord.Bytes.Flags2

Values of the LimitHi, Sys, Reserved_0, Default_Big, and Granularity members in the Bits structure.

HighWord.Bytes.BaseHi

High bits (24–31) of the base address of the segment.

HighWord.Bits

HighWord.Bits.BaseMid

The middle bits (16–23) of the base address of the segment.

HighWord.Bits.Type

The type of segment. This member can be one of the following values:

HighWord.Bits.Dpl

The privilege level of the descriptor. This member is an integer value in the range 0 (most privileged) through 3 (least privileged).

HighWord.Bits.Pres

The present flag. This member is 1 if the segment is present in physical memory or 0 if it is not.

HighWord.Bits.LimitHi

The high bits (16–19) of the address of the last byte in the segment.

HighWord.Bits.Sys

The space that is available to system programmers. This member might be used for marking segments in some system-specific way.

HighWord.Bits.Reserved_0

Reserved.

HighWord.Bits.Default_Big

The 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.

HighWord.Bits.Granularity

The granularity. This member contains 0 if the segment is byte granular, 1 if the segment is page granular.

HighWord.Bits.BaseHi

The 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.

Requirements

Requirement Value
Minimum supported client Windows XP [desktop apps only]
Minimum supported server Windows Server 2003 [desktop apps only]
Header winnt.h (include Windows.h)

See also

Debugging Structures

GetThreadSelectorEntry