SGDT

Store Global Descriptor Table Register

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
0F 01 /0 SGDT m A Valid Valid Store GDTR to m.

Instruction Operand Encoding

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

Description

Stores the content of the global descriptor table register (GDTR) in the destination operand. The destination operand specifies a memory location.

In legacy or compatibility mode, the destination operand is a 6-byte memory location. If the operand-size attribute is 16 bits, the limit is stored in the low 2 bytes and the 24-bit base address is stored in bytes 3-5, and byte 6 is zero-filled. If the operand-size attribute is 32 bits, the 16-bit limit field of the register is stored in the low 2 bytes of the memory location and the 32-bit base address is stored in the high 4 bytes.

In IA-32e mode, the operand size is fixed at 8+2 bytes. The instruction stores an 8byte base and a 2-byte limit.

SGDT is useful only by operating-system software. However, it can be used in application programs without causing an exception to be generated. See "LGDT/LIDT—Load Global/Interrupt Descriptor Table Register" in Chapter 3, Intel® 64 and IA-32 Architectures Software Developer's Manual, Volume 2A, for information on loading the GDTR and IDTR.

Pseudo Code

IF instruction is SGDT
	IF OperandSize = 16
		DEST[0:15] = GDTR(Limit);
		DEST[16:39] = GDTR(Base); (* 24 bits of base address stored *)
		DEST[40:47] = 0;
	ELSE
		IF (32-bit Operand Size)
			DEST[0:15] = GDTR(Limit);
			DEST[16:47] = GDTR(Base); (* Full 32-bit base address stored *)
		FI;
	ELSE
		(* 64-bit Operand Size *)
		DEST[0:15] = GDTR(Limit);
		DEST[16:79] = GDTR(Base); (* Full 64-bit base address stored *)
	FI;
FI;

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Exception Description
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#GP(0) If the memory address is in a non-canonical form.
#UD If the destination operand is a register. If the LOCK prefix is used.
#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
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#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 a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#UD If the destination operand is a register. 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 the destination operand is a register. If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#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 destination is located in a non-writable segment. 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 the destination operand is a register. If the LOCK prefix is used.