sgdt

Store Global Descriptor Table Register

SGDT m

Stores GDT limit and base address to memory.

Details

Stores the current Global Descriptor Table (GDT) register contents (limit and base address) to memory. The format is 2-byte limit followed by 4-byte base in 32-bit mode, or 2-byte limit followed by 8-byte base in 64-bit mode. No flags are affected. This instruction is not privileged and can be executed at any CPL.

Pseudocode Operation

[m + 0:1] ← GDTR.limit
[m + 2:5] ← GDTR.base  // 4 or 8 bytes depending on mode

Example

SGDT [rbp-8]

Encoding

Binary Layout

ModRM

Format System

Opcode 0F 01 /0

Extension System

Operands

dest
Memory operand

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
0F 01 /0	SGDT m	M	Valid Valid		Store GDTR to m.

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 or 32 bits, the 16-bit limit field of the register is stored in the low 2 bytes of the memory location and the 32bit base address is stored in the high 4 bytes. In 64-bit mode, the operand size is fixed at 8+2 bytes. The instruction stores an 8-byte 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 if CR4.UMIP = 0. 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. IA-32 Architecture Compatibility The 16-bit form of the SGDT is compatible with the Intel 286 processor if the upper 8 bits are not referenced. The Intel 286 processor fills these bits with 1s; processor generations later than the Intel 286 processor fill these bits with 0s.

Operation

IF instruction is SGDT
IF OperandSize =16 or OperandSize = 32 (* Legacy or Compatibility Mode *)
THEN
DEST[0:15] := GDTR(Limit);
DEST[16:47] := GDTR(Base); (* Full 32-bit base address stored *)
FI;
ELSE (* 64-bit Mode *)
DEST[0:15] := GDTR(Limit);
DEST[16:79] := GDTR(Base); (* Full 64-bit base address stored *)
FI;
FI;

Flags Affected

None. SGDT—Store Global Descriptor Table Register Vol. 2B 4-628

Exceptions

Protected Mode Exceptions

#UD If the LOCK prefix is used. #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. If CR4.UMIP = 1 and CPL > 0. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while CPL = 3.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used. #GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit.

Virtual-8086 Mode Exceptions

#UD If the LOCK prefix is used. #GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If CR4.UMIP = 1. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #UD If the LOCK prefix is used. #GP(0) If the memory address is in a non-canonical form. If CR4.UMIP = 1 and CPL > 0. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while CPL = 3. SGDT—Store Global Descriptor Table Register Vol. 2B 4-629