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	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). 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.