SGDT

Store Global Descriptor Table Register

참고 사항

Instruction Operand Encoding

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 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(R) 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
                THEN 
                      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.

Protected Mode Exceptions

#UD

• If the destination operand is a register.

• 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 destination operand is a register.

• 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 destination operand is a register.

• 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 destination operand is a register.

• 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.

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