LDS/LES/LFS/LGS/LSS

Load Far Pointer

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
REX + 0F B5 /r LGS r64,m16:64 A Valid N.E. Load GS:r64 with far pointer from memory.
0F B5 /r LGS r32,m16:32 A Valid Valid Load GS:r32 with far pointer from memory.
0F B5 /r LGS r16,m16:16 A Valid Valid Load GS:r16 with far pointer from memory.
REX + 0F B4 /r LFS r64,m16:64 A Valid N.E. Load FS:r64 with far pointer from memory.
0F B4 /r LFS r32,m16:32 A Valid Valid Load FS:r32 with far pointer from memory.
0F B4 /r LFS r16,m16:16 A Valid Valid Load FS:r16 with far pointer from memory.
C4 /r LES r32,m16:32 A Invalid Valid Load ES:r32 with far pointer from memory.
C4 /r LES r16,m16:16 A Invalid Valid Load ES:r16 with far pointer from memory.
REX + 0F B2 /r LSS r64,m16:64 A Valid N.E. Load SS:r64 with far pointer from memory.
0F B2 /r LSS r32,m16:32 A Valid Valid Load SS:r32 with far pointer from memory.
0F B2 /r LSS r16,m16:16 A Valid Valid Load SS:r16 with far pointer from memory.
C5 /r LDS r32,m16:32 A Invalid Valid Load DS:r32 with far pointer from memory.
C5 /r LDS r16,m16:16 A Invalid Valid Load DS:r16 with far pointer from memory.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA ModRM:r/m (r) ModRM:reg (w)

Description

Loads a far pointer (segment selector and offset) from the second operand (source operand) into a segment register and the first operand (destination operand). The source operand specifies a 48-bit or a 32-bit pointer in memory depending on the current setting of the operand-size attribute (32 bits or 16 bits, respectively). The instruction opcode and the destination operand specify a segment register/generalpurpose register pair. The 16-bit segment selector from the source operand is loaded into the segment register specified with the opcode (DS, SS, ES, FS, or GS). The 32-bit or 16-bit offset is loaded into the register specified with the destination operand.

If one of these instructions is executed in protected mode, additional information from the segment descriptor pointed to by the segment selector in the source operand is loaded in the hidden part of the selected segment register.

Also in protected mode, a NULL selector (values 0000 through 0003) can be loaded into DS, ES, FS, or GS registers without causing a protection exception. (Any subsequent reference to a segment whose corresponding segment register is loaded with a NULL selector, causes a general-protection exception (#GP) and no memory reference to the segment occurs.)

In 64-bit mode, the instruction's default operation size is 32 bits. Using a REX prefix in the form of REX.W promotes operation to specify a source operand referencing an 80-bit pointer (16-bit selector, 64-bit offset) in memory. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). See the summary chart at the beginning of this section for encoding data and limits.

Pseudo Code

(* (* 64-BIT_MODE *) *)
IF SS is loaded
	IF SegmentSelector = NULL and ((RPL = 3) or (RPL != 3 and RPL != CPL))
		#GP(0);
	ELSE
		IF descriptor is in non-canonical space
			#GP(0);
		FI;
	ELSE
		IF Segment selector index is not within descriptor table limits or segment selector RPL != CPL or access rights indicate nonwritable data segment or DPL != CPL
			#GP(selector);
		FI;
	ELSE
		IF Segment marked not present
			#SS(selector);
		FI;
	FI;
	SS = SegmentSelector(SRC);
	SS = SegmentDescriptor([SRC]);
ELSE
	IF attempt to load DS, or ES
		#UD;
	ELSE
		IF FS, or GS is loaded with non-NULL segment selector
			IF Segment selector index is not within descriptor table limits or access rights indicate segment neither data nor readable code segment or segment is data or nonconforming-code segment and (RPL > DPL or CPL > DPL)
				#GP(selector);
			FI;
		ELSE
			IF Segment marked not present
				#NP(selector);
			FI;
			SegmentRegister = SegmentSelector(SRC);
			SegmentRegister = SegmentDescriptor([SRC]);
		FI;
	ELSE
		IF FS, or GS is loaded with a NULL selector:
			SegmentRegister = NULLSelector;
			SegmentRegister(DescriptorValidBit) = 0;
			(* Hidden flag; not accessible by software *)
	FI;
	DEST = Offset(SRC);
	(* PROTECTED MODE OR COMPATIBILITY MODE *)
	IF SS is loaded
		IF SegementSelector = NULL
			#GP(0);
		ELSE
			IF Segment selector index is not within descriptor table limits or segment selector RPL != CPL or access rights indicate nonwritable data segment or DPL != CPL
				#GP(selector);
			FI;
		ELSE
			IF Segment marked not present
				#SS(selector);
			FI;
		FI;
		SS = SegmentSelector(SRC);
		SS = SegmentDescriptor([SRC]);
	ELSE
		IF DS, ES, FS, or GS is loaded with non-NULL segment selector
			IF Segment selector index is not within descriptor table limits or access rights indicate segment neither data nor readable code segment or segment is data or nonconforming-code segment and (RPL > DPL or CPL > DPL)
				#GP(selector);
			FI;
		ELSE
			IF Segment marked not present
				#NP(selector);
			FI;
			SegmentRegister = SegmentSelector(SRC) AND RPL;
			SegmentRegister = SegmentDescriptor([SRC]);
		FI;
	ELSE
		IF DS, ES, FS, or GS is loaded with a NULL selector:
			SegmentRegister = NULLSelector;
			SegmentRegister(DescriptorValidBit) = 0;
			(* Hidden flag; not accessible by software *)
	FI;
	DEST = Offset(SRC);
	Real-Address or Virtual-8086 Mode SegmentRegister = SegmentSelector(SRC);
FI;
DEST = Offset(SRC);

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If source operand is not a memory location. If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#NP(selector) If FS, or GS register is being loaded with a non-NULL segment selector and the segment is marked not present.
#SS(0) If a memory operand effective address is non-canonical #SS(Selector) If the SS register is being loaded and the segment is marked not present.
#GP(Selector) If the FS, or GS register is being loaded with a non-NULL segment selector and any of the following is true: the segment selector index is not within descriptor table limits, the memory address of the descriptor is non-canonical, the segment is neither a data nor a readable code segment, or the segment is a data or nonconforming-code segment and both RPL and CPL are greater than DPL. If the SS register is being loaded and any of the following is true: the segment selector index is not within the descriptor table limits, the memory address of the descriptor is non-canon ical, the segment selector RPL is not equal to CPL, the segment is a nonwritable data segment, or DPL is not equal to CPL.
#GP(0) If the memory address is in a non-canonical form. If a NULL selector is attempted to be loaded into the SS register in compatibility mode. If a NULL selector is attempted to be loaded into the SS register in CPL3 and 64-bit mode. If a NULL selector is attempted to be loaded into the SS register in non-CPL3 and 64-bit mode where its RPL is not equal to CPL.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#UD If source operand is not a memory location. If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#UD If source operand is not a memory location. If the LOCK prefix is used.
#SS If a memory operand effective address is outside the SS segment limit.
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Protected Mode Exceptions

Exception Description
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#NP(selector) If DS, ES, FS, or GS register is being loaded with a non-NULL segment selector and the segment is marked not present.
#SS(selector) If the SS register is being loaded and the segment is marked not present.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(selector) If the SS register is being loaded and any of the following is true: the segment selector index is not within the descriptor table limits, the segment selector RPL is not equal to CPL, the segment is a non-writable data segment, or DPL is not equal to CPL. If the DS, ES, FS, or GS register is being loaded with a non-NULL segment selector and any of the following is true: the segment selector index is not within descriptor table limits, the segment is neither a data nor a readable code segment, or the segment is a data or nonconforming-code segment and both RPL and CPL are greater than DPL.
#GP(0) If a NULL selector is loaded into the SS register. 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.
#UD If source operand is not a memory location. If the LOCK prefix is used.