| Opcode* | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description |
| FD | STD | NP | Valid | Valid | Set DF flag. |
| Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
| NP | NA | NA | NA | NA |
Sets the DF flag in the EFLAGS register. When the DF flag is set to 1, string operations decrement the index registers (ESI and/or EDI).
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
DF ← 1;
The DF flag is set. The CF, OF, ZF, SF, AF, and PF flags are unaffected.
| #UD | If the LOCK prefix is used. |