LDS, LES, LFS, LGS, LSS

Load Far Pointer

stableVMJITAOTinstruction

Encodings

Opcode	Instruction	Op/En	64-bit	Compat/Legacy	Description
`C5 /r`	`LDS r16,m16:16`	RM	Invalid	Valid	Load DS:r16 with far pointer from memory.
`C5 /r`	`LDS r32,m16:32`	RM	Invalid	Valid	Load DS:r32 with far pointer from memory.
`0F B2 /r`	`LSS r16,m16:16`	RM	Valid	Valid	Load SS:r16 with far pointer from memory.
`0F B2 /r`	`LSS r32,m16:32`	RM	Valid	Valid	Load SS:r32 with far pointer from memory.
`REX.W + 0F B2 /r`	`LSS r64,m16:64`	RM	Valid	Not encodable	Load SS:r64 with far pointer from memory.
`C4 /r`	`LES r16,m16:16`	RM	Invalid	Valid	Load ES:r16 with far pointer from memory.
`C4 /r`	`LES r32,m16:32`	RM	Invalid	Valid	Load ES:r32 with far pointer from memory.
`0F B4 /r`	`LFS r16,m16:16`	RM	Valid	Valid	Load FS:r16 with far pointer from memory.
`0F B4 /r`	`LFS r32,m16:32`	RM	Valid	Valid	Load FS:r32 with far pointer from memory.
`REX.W + 0F B4 /r`	`LFS r64,m16:64`	RM	Valid	Not encodable	Load FS:r64 with far pointer from memory.
`0F B5 /r`	`LGS r16,m16:16`	RM	Valid	Valid	Load GS:r16 with far pointer from memory.
`0F B5 /r`	`LGS r32,m16:32`	RM	Valid	Valid	Load GS:r32 with far pointer from memory.
`REX.W + 0F B5 /r`	`LGS r64,m16:64`	RM	Valid	Not encodable	Load GS:r64 with far pointer from memory.

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.

RM

modrm.reg escrituraModRM byte, reg field (bits 5-3)
modrm.rm lecturaModRM byte, r/m field (bits 2-0); with the SIB byte and the displacement when the mod field asks for them

Measured cost

Loading measurements from arch-data...

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/general-purpose 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.

Operation

64-BIT_MODE
    IF SS is loaded
          THEN

             IF SegmentSelector = NULL and ( (RPL = 3) or
                       (RPL  3 and RPL  CPL) )

                      THEN #GP(0);
                ELSE IF descriptor is in non-canonical space


                       THEN #GP(selector); 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

                       THEN #GP(selector); FI;
                ELSE IF Segment marked not present

                       THEN #SS(selector); FI;
                FI;
                SS := SegmentSelector(SRC);
                SS := SegmentDescriptor([SRC]);
    ELSE IF attempt to load DS, or ES
          THEN #UD;
    ELSE IF FS, or GS is loaded with non-NULL segment selector
          THEN 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)

                       THEN #GP(selector); FI;
                ELSE IF Segment marked not present

                       THEN #NP(selector); FI;
                FI;
                SegmentRegister := SegmentSelector(SRC) ;
                SegmentRegister := SegmentDescriptor([SRC]);
          FI;
    ELSE IF FS, or GS is loaded with a NULL selector:
          THEN
                SegmentRegister := NULLSelector;
                SegmentRegister(DescriptorValidBit) := 0; FI; (* Hidden flag;

                       not accessible by software *)
    FI;
    DEST := Offset(SRC);

PREOTECTED MODE OR COMPATIBILITY MODE;
    IF SS is loaded
          THEN

             IF SegementSelector = NULL

                       THEN #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

                       THEN #GP(selector); FI;
                ELSE IF Segment marked not present

                       THEN #SS(selector); FI;
                FI;
                SS := SegmentSelector(SRC);
                SS := SegmentDescriptor([SRC]);
    ELSE IF DS, ES, FS, or GS is loaded with non-NULL segment selector
          THEN 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)

                       THEN #GP(selector); FI;
                ELSE IF Segment marked not present


                      THEN #NP(selector); FI;
                FI;
                SegmentRegister := SegmentSelector(SRC) AND RPL;
                SegmentRegister := SegmentDescriptor([SRC]);
          FI;
    ELSE IF DS, ES, FS, or GS is loaded with a NULL selector:
          THEN
                SegmentRegister := NULLSelector;
                SegmentRegister(DescriptorValidBit) := 0; FI; (* 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

Protected mode

`#UD`	If source operand is not a memory location. If the LOCK prefix is used.
`#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.
`#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.
`#SS(0)`	If a memory operand effective address is outside the SS segment limit.
`#SS(selector)`	If the SS register is being loaded and the segment is marked not present.
`#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. #PF(fault-code) If a page fault occurs.
`#AC(0)`	If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

Real address mode

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

Virtual-8086 mode

`#UD`	If source operand is not a memory location. If the LOCK prefix is used.
`#GP(0)`	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
`#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.

Compatibility mode

Same exceptions as in protected mode.

64-bit mode

`#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.
`#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-canonical, the segment selector RPL is not equal to CPL, the segment is a nonwritable data segment, or DPL is not equal to CPL.
`#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.
`#NP(selector)`	If FS, or GS register is being loaded with a non-NULL segment selector and the segment is marked not present. #PF(fault-code) If a page fault occurs.
`#AC(0)`	If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
`#UD`	If source operand is not a memory location. If the LOCK prefix is used.

LDS, LES, LFS, LGS, LSS

Encodings

Operand encoding

RM

Measured cost

Description#

Operation#

Flags affected#

Exceptions

Sources

Description

Operation

Flags affected