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 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.