LGDT/LIDT

Load Global/Interrupt Descriptor Table Register

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
0F 01 /3 LIDT m16&64 A Valid N.E. Load m into IDTR.
0F 01 /2 LGDT m16&64 A Valid N.E. Load m into GDTR.
0F 01 /3 LIDT m16&32 A N.E. Valid Load m into IDTR.
0F 01 /2 LGDT m16&32 A N.E. Valid Load m into GDTR.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA ModRM:r/m (r)

Description

Loads the values in the source operand into the global descriptor table register (GDTR) or the interrupt descriptor table register (IDTR). The source operand specifies a 6-byte memory location that contains the base address (a linear address) and the limit (size of table in bytes) of the global descriptor table (GDT) or the interrupt descriptor table (IDT). If operand-size attribute is 32 bits, a 16-bit limit (lower 2 bytes of the 6-byte data operand) and a 32-bit base address (upper 4 bytes of the data operand) are loaded into the register. If the operand-size attribute is 16 bits, a 16-bit limit (lower 2 bytes) and a 24-bit base address (third, fourth, and fifth byte) are loaded. Here, the high-order byte of the operand is not used and the high-order byte of the base address in the GDTR or IDTR is filled with zeros.

The LGDT and LIDT instructions are used only in operating-system software; they are not used in application programs. They are the only instructions that directly load a linear address (that is, not a segment-relative address) and a limit in protected mode. They are commonly executed in real-address mode to allow processor initialization prior to switching to protected mode.

In 64-bit mode, the instruction's operand size is fixed at 8+2 bytes (an 8-byte base and a 2-byte limit). See the summary chart at the beginning of this section for encoding data and limits.

See "SGDT—Store Global Descriptor Table Register" in Chapter 4, Intel® 64 andIA-32 Architectures Software Developer's Manual, Volume 2B, for information on storing the contents of the GDTR and IDTR.

Pseudo Code

IF Instruction is LIDT
	IF OperandSize = 16
		IDTR(Limit) = SRC[0:15];
		IDTR(Base) = SRC[16:47] AND 00FFFFFFH;
	ELSE
		IF 32-bit Operand Size
			IDTR(Limit) = SRC[0:15];
			IDTR(Base) = SRC[16:47];
		FI;
	ELSE
		IF 64-bit Operand Size (* In 64-Bit Mode *)
			IDTR(Limit) = SRC[0:15];
			IDTR(Base) = SRC[16:79];
		FI;
	FI;
ELSE
	(* Instruction is LGDT *)
	IF OperandSize = 16
		GDTR(Limit) = SRC[0:15];
		GDTR(Base) = SRC[16:47] AND 00FFFFFFH;
	ELSE
		IF 32-bit Operand Size
			GDTR(Limit) = SRC[0:15];
			GDTR(Base) = SRC[16:47];
		FI;
	ELSE
		IF 64-bit Operand Size (* In 64-Bit Mode *)
			GDTR(Limit) = SRC[0:15];
			GDTR(Base) = SRC[16:79];
		FI;
	FI;
FI;

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Exception Description
#PF(fault-code) If a page fault occurs.
#UD If source operand is not a memory location. If the LOCK prefix is used.
#GP(0) If the current privilege level is not 0. If the memory address is in a non-canonical form.
#SS(0) If a memory address referencing the SS segment is in a non-canonical form.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#GP If the current privilege level is not 0.
#GP(0) The LGDT and LIDT instructions are not recognized in virtual8086 mode.
#UD If source operand is not a memory location. If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#SS If a memory operand effective address is outside the SS segment limit.
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#UD If source operand is not a memory location. If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If the current privilege level is not 0. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register is used to access memory and it contains a NULL segment selector.
#UD If source operand is not a memory location. If the LOCK prefix is used.