3-380
INSTRUCTION SET REFERENCE
LTRLoad Task Register
Description
This instruction loads the source operand into the segment selector field of the task register. The
source operand (a general-purpose register or a memory location) contains a segment selector
that points to a task state segment (TSS). After the segment selector is loaded in the task register,
the processor uses the segment selector to locate the segment descriptor for the TSS in the global
descriptor table (GDT). It then loads the segment limit and base address for the TSS from the
segment descriptor into the task register. The task pointed to by the task register is marked busy,
but a switch to the task does not occur.
The LTR instruction is provided for use in operating-system software; it should not be used in
application programs. It can only be executed in protected mode when the CPL is 0. It is
commonly used in initialization code to establish the first task to be executed.
The operand-size attribute has no effect on this instruction.
Operation
IF SRC(Offset) > descriptor table limit OR IF SRC(type)
?
global
THEN #GP(segment selector);
FI;
Read segment descriptor;
IF segment descriptor is not for an available TSS THEN #GP(segment selector); FI;
IF segment descriptor is not present THEN #NP(segment selector);
TSSsegmentDescriptor(busy)
<
1;
(* Locked read-modify-write operation on the entire descriptor when setting busy flag *)
TaskRegister(SegmentSelector)
<
SRC;
TaskRegister(SegmentDescriptor)
<
TSSSegmentDescriptor;
Flags Affected
None.
Opcode
Instruction
Description
0F 00 /3
LTR r/m16
Load r/m16 into task register
