STD

Set Direction Flag

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
FD STD A Valid Valid Set DF flag.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A 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 registers (ESI and/or EDI).

This instruction's operation is the same in non-64-bit modes and 64-bit mode.

Pseudo Code

DF = 1;

Flags Affected

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

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.