CPU Register - DF Control Flag
The direction flag (DF, located in bit 10 of the EFLAGS register) controls string instructions.
Setting the DF flag causes the string instructions to auto-decrement (to process strings from high addresses to low addresses).
Clearing the DF flag causes the string instructions to auto-increment (process strings from low addresses to high addresses). The STD and CLD instructions set and clear the DF flag, respectively.