STD (Set Direction Flag)

Opcodes

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

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
NP NA NA NA NA

Description

Sets the DF flag in the EFLAGS register. When the DF flag is set to 1, string operations decrement the index regis-ters (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.