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Descriptors Window Introduction

The Descriptors windows are used to examine and modify descriptor table entries. The elements of a Descriptors window include the title bar, data area, and option tabs for selecting the descriptor table type. The Descriptors window can be opened by selecting View|Descriptors on the menu bar or by clicking on the Descriptors icon on the toolbar. The GDT Descriptors table opens automatically.

 

GDT Descriptors window

Window Structure

Offset Column

The Offset column lists the value of a selector index field within a segment register that points to a descriptor. A selector index is the decimal entry number multiplied by 8 and displayed as a hexadecimal value.

Type Column

The Type column lists the descriptor type. Code and data descriptor types include abbreviations that define the set/not-set state of their status bits.

  • Code descriptor types are listed with abbreviations for Conforming (C), Readable (R), and Accessed (A) status bits. An exclamation mark (!) precedes an abbreviation if the bit is cleared (e.g., !A=Segment has not been accessed).

  • Data descriptor types are listed with abbreviations for Expand-down (E), Writable (W), and Accessed (A) status bits. An exclamation mark (!) precedes an abbreviation if the bit is cleared (e.g., !E=Expand-up segment).

  • Task State Segment (TSS) descriptor types are listed as 16- and 32-bit TSS and with the word Busy when the TSS is not available.

  • Gate descriptor types are listed as 16- or 32-bit call-gates, 16 or 32-bit interrupt-gates, task-gates, and 16- or 32-bit trap-gates.

Note: Status is not defined for LDT, task-gate, call-gate, and IDT descriptor types. TSS types may include a Busy status.

Attributes Column

The Attributes column defines the Descriptor Privilege Level (DPL) and the Present (P) bit for all descriptors. Other attributes, defined for certain descriptor types, are identified below.

  • The Granularity (G) bit is defined for code, data, TSS, and LDT descriptors (i.e., segment limit is G=page granular or !G=byte granular).

  • The operand/address-mode default size is defined for code (D) and data (B) descriptors (i.e., operand/address-mode is D/B=32-bit or !D/B= 16-bit).

  • The Available (Avl) bit is defined for code, data, and TSS descriptors (i.e., segment is Avl=available or !Avl=not available).

  • The Dword (Doubleword) count is defined for call-gate descriptors.

Values Column

The Values column lists the base address and limit for code, data, TSS, and LDT (table) descriptors, or the selector and offset for call, interrupt, and trap-gate descriptors; it lists the selector (only) for task-gate descriptors.

  • Base defines the location of a segment within the 4 Gbytes of physical address space. A base address is displayed as an 8-digit hexadecimal value.

  • Limit is a 20-bit value representing the size of the memory segment. A limit is displayed as a 5-digit hexadecimal value.

  • Interrupt and trap gate descriptors use a 16-bit selector and a 32-bit offset as destination fields that point to the start of an interrupt or trap routine. A selector is displayed as a 4-digit hexadecimal value; offsets are displayed as 8-digit hexadecimal values.

Note: Task gate descriptors use only the selector field to refer to a TSS.

Tabs

The tabs at the bottom of the window are used to select the descriptor tables to display in the window. The title bar changes to reflect the selected descriptor table and shows the base and limit values of that table.

  • Select the GDT tab to view the status of or modify the contents of a Global Descriptor Table (GDT) entry.

  • Select the IDT tab to view the status of or modify the contents of an Interrupt Descriptor Table (IDT) entry.

  • Select the LDT tab to view the status of or modify the contents of a Local Descriptor Table (LDT) entry.

  • Select the LDTR tab to view the status of or modify the contents of a Local Descriptor Table Register (LDTR) entry.

 The LDT or LDTR option tabs may not be enabled in all situations. The LDTR tab displays the currently active local descriptor table based upon the LDTR register value. The LDT tab is used to display any local descriptor table that is referenced in the GDT. The LDT tab works only when an LDT entry is selected in the GDT display.