The LTR

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

instruction stands for load task register and is used in

operating systems An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

that support multitasking. LTR is supported only in

protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...

and

long mode In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mod ...

, not in

real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...

virtual 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running ...

. It must be executed when the Current Privilege Level (CPL) is 0, and therefore cannot be used by application programs. LTR loads the special x86 task register with a segment selector that points to a task state segment (TSS). After executing the LTR instruction, the TSS pointed to by the argument is marked busy, but no hardware task switch occurs. The opposite of the LTR instruction is the STR instruction, which means store task register and copies the value of the task register to the specified location. Note that the x86 task register is only accessible directly through the LTR and STR instructions.

References

X86 instructions {{Tech-stub