# STR — Store Task Register

Opcode | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description
--- | --- | --- | --- | --- | ---
0F 00 /1 | STR r/m16 | M | Valid | Valid | Stores segment selector from TR in r/m16.

## Instruction Operand Encoding ¶

Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | ---
M | ModRM:r/m (w) | N/A | N/A | N/A

## Description ¶

Stores the segment selector from the task register (TR) in the destination operand. The destination operand can be a general-purpose register or a memory location. The segment selector stored with this instruction points to the task state segment (TSS) for the currently running task.

When the destination operand is a 32-bit register, the 16-bit segment selector is copied into the lower 16 bits of the register and the upper 16 bits of the register are cleared. When the destination operand is a memory location, the segment selector is written to memory as a 16-bit quantity, regardless of operand size.

In 64-bit mode, operation is the same. The size of the memory operand is fixed at 16 bits. In register stores, the 2-byte TR is zero extended if stored to a 64-bit register.

The STR instruction is useful only in operating-system software. It can only be executed in protected mode.

## Operation ¶

    DEST := TR(SegmentSelector);

## Flags Affected ¶

None.

## Protected Mode Exceptions ¶

#GP(0) | If the destination is a memory operand that is located in a non-writable segment or if the 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.
If CR4.UMIP = 1 and CPL > 0.
#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 while the current privilege level is 3.
#UD | If the LOCK prefix is used.

## Real-Address Mode Exceptions ¶

#UD | The STR instruction is not recognized in real-address mode.

## Virtual-8086 Mode Exceptions ¶

#UD | The STR instruction is not recognized in virtual-8086 mode.

## Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

## 64-Bit Mode Exceptions ¶

#GP(0) | If the memory address is in a non-canonical form.
If CR4.UMIP = 1 and CPL > 0.
#SS(0) | If the stack address is in a non-canonical form.
#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 the LOCK prefix is used.