LDT_ENTRY

Overview

Group

Quick Info

Windows NT
Yes
Win95
Yes
Win32s
Yes
Import Library
-
Header File
winnt.h
Unicode
No
Platform Notes
None

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 (1623) 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
	Read-write data segment
	Unused segment
	Read-write expand-down data segment
	Execute-only code segment
	Executable-readable code segment
	Execute-only "conforming" code segment
	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 (1619) 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 (2431) of the base address of the segment.

The members of the Bytes structure are as follows:

Member	Contents
BaseMid	Middle bits (1623) 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 (2431) 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.