# STD — Set Direction Flag

Opcode | Instruction | Op/En | 64-bit Mode | Compat/Leg Mode | Description
--- | --- | --- | --- | --- | ---
FD | STD | ZO | Valid | Valid | Set DF flag.

## Instruction Operand Encoding ¶

Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | ---
ZO | N/A | N/A | N/A | N/A

## Description ¶

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.

## Operation ¶

    DF := 1;

## Flags Affected ¶

The DF flag is set. The CF, OF, ZF, SF, AF, and PF flags are unaffected.

## Exceptions (All Operating Modes) ¶

#UD If the LOCK prefix is used.