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STD
Henk-Jan Lebbink edited this page Jun 4, 2026
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STD — Set Direction Flag
| Opcode | Instruction | Op/ En | 64-bit Mode | Compat/ Leg Mode | Description |
| FD | STD | ZO | Valid | Valid | Set DF flag. |
| Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
| ZO | N/A | N/A | N/A | N/A |
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). Operation is the same in all modes.
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.
Source: Intel® 64 and IA-32 Architectures Software Developer's Manual, Combined Volumes (Order Number 325462-091US, March 2026)
Generated: 7-6-2026