The MOV instruction copies the second operand to the first operand.
If the destination operand is a segment register (DS, ES, SS, etc.), then data from a descriptor is also loaded into the shadow portion of the register. The data for the register is obtained from the descriptor table entry for the selector given. A null selector (values 0000-0003) can be loaded into the DS, ES, FS, and GS registers without causing an exception; however, use of these registers causes a #GP(0) exception, and no memory reference occurs.
A MOV into SS instruction inhibits all interrupts until after the processing of the next instruction (which should be a MOV into ESP instruction).
Loading a segment register under Protected Mode results in special checks
and actions, as described in the following listing: IF SS is loaded;
THEN
IF selector is null THEN #GP(0);
Selector index must be within its descriptor table limits else #GP(selector);
Selector's RPL must equal CPL else #GP(selector);
AR byte must indicate a writable data segment else #GP(selector);
DPL in the AR byte must equal CPL else #GP(selector);
Segment must be marked present else #SS(selector);
Load SS with selector;
Load SS with descriptor.
FI;
IF DS, ES, FS or GS is loaded with non-null selector;
THEN
Selector index must be within its descriptor table limits
else #GP(selector);
AR byte must indicate data or readable code segment else #GP(selector);
IF data or nonconforming code segment
THEN both the RPL and the CPL must be less than or equal to DPL in AR
byte;
ELSE #GP(selector);
FI;
Segment must be marked present else #NP(selector);
Load segment register with selector;
Load segment register with descriptor;
FI;
IF DS, ES, FS or GS is loaded with a null selector;
THEN
Load segment register with selector;
Clear descriptor valid bit;
FI;