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Volume 3: General-Purpose and System Instructions - Stanford ...

Volume 3: General-Purpose and System Instructions - Stanford ...

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24594 Rev. 3.10 February 2005 AMD64 TechnologyLTRLoad Task RegisterLoads the specified segment selector into the visible portion of the task register (TR).The processor uses the selector to locate the descriptor for the TSS in the globaldescriptor table. It then loads this descriptor into the hidden portion of TR. The TSSdescriptor in the GDT is marked busy, but no task switch is made.If the source oper<strong>and</strong> is null, a general protection exception (#GP) is generated.In legacy <strong>and</strong> compatibility modes, the TSS descriptor is 8 bytes long <strong>and</strong> contains a32-bit base address.In 64-bit mode, the instruction references a 64-bit descriptor to load a 64-bit baseaddress. The TSS type (09H) is redefined in 64-bit mode for use as the 16-byte TSSdescriptor.This instruction must be executed in protected mode when the current privilege levelis 0. It is only provided for use by operating system software.The oper<strong>and</strong> size attribute has no effect on this instruction.LTR is a serializing instruction.Mnemonic Opcode DescriptionLTR reg/mem16 0F 00 /3 Load the 16-bit segment selector into the task register <strong>and</strong> load the TSSdescriptor from the GDT.Related <strong>Instructions</strong>LGDT, LIDT, LLDT, STR, SGDT, SIDT, SLDTrFLAGS AffectedNoneLTR 327

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