x86 Instruction Set Reference

STD - Set Direction Flag

Opcode Mnemonic Description
FD STD Set DF flag.
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 
DF = 1;
Flags affected

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