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.)
Operation
IF ProtectedMode
THEN IF SS is loaded
THEN IF SegementSelector null
THEN #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
THEN #GP(selector);
FI;
ELSE IF Segment marked not present
THEN #SS(selector);
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 both RPL and CPL > DPL)
THEN #GP(selector);
FI;
ELSE IF Segment marked not present
THEN #NP(selector);
FI;
SegmentRegister
SegmentSelector(SRC) AND RPL;
SegmentRegister
SegmentDescriptor([SRC]);
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;
FI;
IF (Real-Address or Virtual-8086 Mode)
THEN
SegmentRegister
SegmentSelector(SRC);
FI;
DEST Offset(SRC);
Flags Affected
None.
Protected Mode Exceptions
#UD - If source operand is not a memory location.
#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
nonwritable 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 Exceptions
#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.
Virtual-8086 Mode Exceptions
#UD - If source operand is not a memory location.
#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.
For details, see Volume 2A and Volume 2B of the Intel(R)
64 and IA-32 Intel Architecture Software Developer's Manual. For
the latest updates on the instruction set information, go to the
web site.
null
CPL