查看“Global Descriptor Table”的源代码

“全局描述符表”（“GDT”）是特定于[[IA32_体系结构| IA-32]和[[X86-64 | X86-64]]体系结构的二进制数据结构。它包含告诉CPU关于内存[[分段|分段]]的条目。 类似的[[中断描述符表]]包含[[任务]]和[[中断|中断]]描述符。

建议阅读[[GDT教程]]。

== GDTR ==

“GDT”由“GDTR”寄存器中的值指向。 这是使用“LGDT”汇编指令加载的，该指令的参数是指向“GDT描述符”结构的指针：

{| class="wikitable"
|+ GDT Descriptor (GDTR)
!style="width: 66%; text-align: left;" |79 (64-bit mode) <br> 48 (32-bit mode)&nbsp;&nbsp;&nbsp;<span style="float: right;">16</span>
!style="width: 34%; text-align: left; vertical-align: bottom;" |15&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|-
|'''Offset'''<br>63 (64-bit mode) <br> 31 (32-bit mode)&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|'''Size'''<br><br>15&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|}
* '''Size:''' 表的大小（以字节为单位）减去1。之所以发生此减法，是因为“Size”的最大值为65535，而“GDT”的长度最多为65536字节（8192个条目）。此外，任何“GDT”的大小都不能为0字节。
* '''Offset:''' “GDT”的线性地址（不是物理地址，分页适用）。

请注意，“LGDT”加载的数据量在32位和64位模式下有所不同，偏移量在32位模式下为4字节长，在64位模式下为8字节长。

有关更多信息，请参见“第2.4节”。1：《英特尔软件开发人员手册》第3-A卷的“全局描述符表寄存器（GDTR）”和“图2-6：内存管理寄存器”。

== 表格 ==

“GDT”中的条目长度为8字节，形成如下表格：

{|class="wikitable"
|+ Global Descriptor Table
! Address !! Content
|-
| GDTR Offset + 0 || Null
|-
| GDTR Offset + 8 || Entry 1
|-
| GDTR Offset + 16 || Entry 2
|-
| GDTR Offset + 24 || Entry 3
|- style="text-align: center;"
| '''...''' || '''...'''
|}

“'GDT'”（条目0）中的第一个条目应始终为空，并应使用后续条目。

“段选择器”可访问表中的条目，它们通过汇编指令或硬件功能（如“[[Interrupts]]”加载到“[[Segmentation]]”寄存器中。

== 段描述符 ==

表中的每个条目都有一个复杂的结构：
{| class="wikitable"
|+ Segment Descriptor
!style="width: 20%; text-align: left;"|63&nbsp;&nbsp;&nbsp;<span style="float: right;">56</span>
!style="width: 12.5%; text-align: left;"|55&nbsp;&nbsp;&nbsp;<span style="float: right;">52</span>
!style="width: 12.5%; text-align: left;"|51&nbsp;&nbsp;&nbsp;<span style="float: right;">48</span>
!style="width: 25%; text-align: left;"|47&nbsp;&nbsp;&nbsp;<span style="float: right;">40</span>
!style="width: 25%; text-align: left;"|39&nbsp;&nbsp;&nbsp;<span style="float: right;">32</span>
|-
|'''Base'''<br>31&nbsp;&nbsp;&nbsp;<span style="float: right;">24</span>
|'''Flags'''<br>3&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|'''Limit'''<br>19&nbsp;&nbsp;&nbsp;<span style="float: right;">16</span>
|'''Access Byte'''<br>7&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|'''Base'''<br>23&nbsp;&nbsp;&nbsp;<span style="float: right;">16</span>
|-
!colspan=3 style="text-align: left;"|31 &nbsp;&nbsp;<span style="float: right;">16</span>
!colspan=2 style="text-align: left;"|15 &nbsp;&nbsp;<span style="float: right;">0</span>
|-
|colspan=3|'''Base'''<br>15&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|colspan=2|'''Limit'''<br>15&nbsp;&nbsp;&nbsp;<span style="float: right;">0</span>
|}

* '''Base:''' 一个32位的值，包含段开始的线性地址。
* '''Limit:''' 一个20位的值，以1字节单位或4KB页面表示最大可寻址单元。 因此，如果选择页面粒度并将“限制”值设置为0xFFFFF，则该段将在32位模式下跨越整个4 GiB地址空间。

在64位模式下，忽略“Base”和“Limit”值，每个描述符覆盖整个线性地址空间，而不管它们设置为什么。

有关更多信息，请参见“第3.4节”。《英特尔软件开发人员手册》第3-A卷第5部分：段描述符“”和“图3-8：段描述符”。

{| class="wikitable"
|+ Access Byte
|-
!style="width: 12.5%;"|7
!style="width: 12.5%;"|6
!style="width: 12.5%;"|5
!style="width: 12.5%;"|4
!style="width: 12.5%;"|3
!style="width: 12.5%;"|2
!style="width: 12.5%;"|1
!style="width: 12.5%;"|0
|-
|'''Pr''' ||colspan=2|'''Privl''' || '''S''' || '''Ex''' || '''DC''' || '''RW''' || '''Ac'''
|}

* '''Pr:''' 当前位。 允许条目引用有效段。 Must be set ('''1''') for any valid segment.
* '''Privl:''' 描述符特权级别字段。 包含段的[[Security#Rings | CPU特权级别]]。 '''0''' = highest privilege (kernel), '''3''' = lowest privilege (user applications).
* '''S:''' 描述符类型位。 If clear ('''0''') the descriptor defines a system segment (eg. a [[Task_State_Segment|Task State Segment]]). 如果设置（'''1''），则定义代码或数据段。
* '''Ex:''' Executable bit. If clear ('''0''') the descriptor defines a data segment. If set ('''1''') it defines a code segment which can be executed from.
* '''DC:''' Direction bit/Conforming bit.
  ** For data selectors: Direction bit. If clear ('''0''') the segment grows up. If set ('''1''') the segment [[Expand_Down|grows down]], ie. the '''Offset''' has to be greater than the '''Limit'''.
  ** For code selectors: Conforming bit.
  *** If clear ('''0''') code in this segment can only be executed from the ring set in '''Privl'''.
  *** If set ('''1''') code in this segment can be executed from an equal or lower privilege level. For example, code in ring 3 can far-jump to ''conforming'' code in a ring 2 segment. The '''Privl''' field represent the highest privilege level that is allowed to execute the segment. For example, code in ring 0 cannot far-jump to a conforming code segment where '''Privl''' is 2, while code in ring 2 and 3 can.  请注意，特权级别保持不变，即。 a far-jump from ring 3 to a segment with a '''Privl''' of 2 remains in ring 3 after the jump.
* '''RW:''' Readable bit/Writable bit.
  ** For code segments: Readable bit. If clear ('''0'''), read access for this segment is not allowed. If set ('''1''') read access is allowed. 决不允许对代码段进行写访问。
  ** 对于数据段：可写位。 If clear ('''0'''), write access for this segment is not allowed. If set ('''1''') write access is allowed. Read access is always allowed for data segments.
* '''Ac:''' Accessed bit. Best left clear ('''0'''), the CPU will set it when the segment is accessed.

{| class="wikitable"
|+ Flags
|-
!style="width: 25%;"|3
!style="width: 25%;"|2
!style="width: 25%;"|1
!style="width: 25%;"|0
|-
|'''Gr''' || '''Sz''' || '''L'''
|Reserved
|}

* '''Gr:''' Granularity flag, indicates the size the '''Limit''' value is scaled by. If clear ('''0'''), the '''Limit''' is in 1 Byte blocks (byte granularity). If set ('''1'''), the '''Limit''' is in 4 KiB blocks (page granularity).
* '''Sz:''' Size flag. If clear ('''0'''), the descriptor defines a 16-bit protected mode segment. If set ('''1''') it defines a 32-bit protected mode segment. A GDT can have both 16-bit and 32-bit selectors at once.
* '''L:''' Long-mode code flag. If set ('''1'''), the descriptor defines a 64-bit code segment. When set, '''Sz''' should always be clear. For any other type of segment (other code types or any data segment), it should be clear ('''0''').

== 另见 ==
=== 文章 ===
* [[GDT Tutorial]]
* [[Getting to Ring 3]]
* [[Segmentation]]
* http://www.osdever.net/tutorials/view/the-world-of-protected-mode - how to set up GDT in assembler

=== 外部参照 ===

* [http://files.osdev.org/mirrors/geezer/os/pm.htm Protected Mode tutorial]
* [http://www.intel.com/design/processor/manuals/253668.pdf Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A:. System Programming Guide, Part 1 (order number 253668)] chapter 2.4[[Category:X86 CPU]]
  [[de:Global Descriptor Table]]