| 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. |
| Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
| M | ModRM:r/m (w) | NA | NA | NA |
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.
DEST ← TR(SegmentSelector);
None.
| #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. |
| #UD | The STR instruction is not recognized in real-address mode. |
| #UD | The STR instruction is not recognized in virtual-8086 mode. |
Same exceptions as in protected mode.
| #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. |