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SLDT

Store Local Descriptor Table Register

stableVMJITAOTinstruction

Encodings

OpcodeInstructionOp/En64-bitCompat/LegacyDescription
0F 00 /0SLDT r/m16MValidValidStores segment selector from LDTR in r/m16.

Operand encoding

Each mode is a value of the Op/En column above. It says which field of the encoded instruction carries each operand, in the order they are written, and whether the instruction reads it, writes it or both.

M

  1. modrm.rm escrituraModRM byte, r/m field (bits 2-0); with the SIB byte and the displacement when the mod field asks for them

Measured cost

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Description

Stores the segment selector from the local descriptor table register (LDTR) in the destination operand. The destination operand can be a general-purpose register or a memory location. The segment selector stored with this instruction points to the segment descriptor (located in the GDT) for the current LDT. This instruction can only be executed in protected mode.

Outside IA-32e mode, when the destination operand is a 32-bit register, the 16-bit segment selector is copied into the low-order 16 bits of the register. The high-order 16 bits of the register are cleared for the Pentium 4, Intel Xeon, and P6 family processors. They are undefined for Pentium, Intel486, and Intel386 processors. When the destination operand is a memory location, the segment selector is written to memory as a 16-bit quantity, regardless of the operand size.

In compatibility mode, when the destination operand is a 32-bit register, the 16-bit segment selector is copied into the low-order 16 bits of the register. The high-order 16 bits of the register are cleared. When the destination operand is a memory location, the segment selector is written to memory as a 16-bit quantity, regardless of the operand size.

In 64-bit mode, using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). The behavior of SLDT with a 64-bit register is to zero-extend the 16-bit selector and store it in the register. If the destination is memory and operand size is 64, SLDT will write the 16-bit selector to memory as a 16-bit quantity, regardless of the operand size.

Operation

DEST := LDTR(SegmentSelector);

Flags affected

None.

Exceptions

Protected mode
#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.
#UDIf the LOCK prefix is used.
Real address mode
#UDThe SLDT instruction is not recognized in real-address mode.
Virtual-8086 mode
#UDThe SLDT instruction is not recognized in virtual-8086 mode.
Compatibility mode
Same exceptions as in protected mode.
64-bit mode
#SS(0)If a memory address referencing the SS segment is in a non-canonical form.
#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.
#UDIf the LOCK prefix is used.

Sources