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RDFSBASE

Reads the current base address of the %fs segment and stores it into the specified destination register.

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

This instruction is only available when the processor is operating in 64-bit mode. It is NOT supported in compatibility mode.

The instruction ignores the current value of the FS segment selector in the segment register; it retrieves the base address directly from the hidden portion of the descriptor cache (the base address specified in the Model Specific Register IA32_FS_BASE). If the processor is not in 64-bit mode, executing this instruction SHALL result in an invalid opcode exception.