About
The direction flag (DF, located in bit 10 of the EFLAGS register) controls string instructions.
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Management
Set
Setting the DF flag causes the string instructions to auto-decrement (to process strings from high addresses to low addresses).
Clear
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.