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AMD64 Technology 24594 Rev. 3.10 February 2005LAHFLoad Status Flags into AH RegisterLoads the lower 8 bits of the rFLAGS register, including sign flag (SF), zero flag (ZF),auxiliary carry flag (AF), parity flag (PF), and carry flag (CF), into the AH register.The instruction sets the reserved bits 1, 3, and 5 of the rFLAGS register to 1, 0, and 0,respectively, in the AH register.The LAHF instruction can only be executed in 64-bit mode if supported by theprocessor implementation. Check the status of ECX bit 0 returned by CPUIDextended function 8000_0001h to verify that the processor supports LAHF in 64-bitmode.Mnemonic Opcode DescriptionLAHF 9F Load the SF, ZF, AF, PF, and CF flags into the AH register.Related InstructionsSAHFrFLAGS AffectedExceptionsException RealVirtual8086 Protected Cause of ExceptionInvalid opcode, #UD X This instruction is not supported in 64-bit mode, as indicated by ECXbit 0 returned by CPUID standard function 8000_0001h.178 LAHF
24594 Rev. 3.10 February 2005 AMD64 TechnologyLDSLESLFSLGSLSSLoad Far PointerLoads a far pointer from a memory location (second operand) into a segment register(mnemonic) and general-purpose register (first operand). The instruction stores the16-bit segment selector of the pointer into the segment register and the 16-bit or 32-bit offset portion into the general-purpose register. The operand-size attributedetermines whether the pointer is 32-bit or 48-bit.These instructions load associated segment-descriptor information into the hiddenportion of the specified segment register.Using LDS or LES in 64-bit mode generates an invalid-opcode exception.Executing LFS, LGS, or LSS with a 64-bit operand size only loads a 32-bit generalpurpose register and the specified segment register.Mnemonic Opcode DescriptionLDS reg16, mem16:16 C5 /rLDS reg32, mem16:32 C5 /rLES reg16, mem16:16 C4 /rLES reg32, mem16:32 C4 /rLoad DS:reg16 with a far pointer from memory.(Invalid in 64-bit mode.)Load DS:reg32 with a far pointer from memory.(Invalid in 64-bit mode.)Load ES:reg16 with a far pointer from memory.(Invalid in 64-bit mode.)Load ES:reg32 with a far pointer from memory.(Invalid in 64-bit mode.)LFS reg16, mem16:16 0F B4 /r Load FS:reg16 with a far pointer from memory.LFS reg32, mem16:32 0F B4 /r Load FS:reg32 with a far pointer from memory.LGS reg16, mem16:16 0F B5 /r Load GS:reg16 with a far pointer from memory.LGS reg32, mem16:32 0F B5 /r Load GS:reg32 with a far pointer from memory.LSS reg16, mem16:16 0F B2 /r Load SS:reg16 with a far pointer from memory.LSS reg32, mem16:32 0F B2 /r Load SS:reg32 with a far pointer from memory.LxS 179
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