When it is set to 0 (using the clear-direction-flag instruction CLD), it means that instructions that autoincrement the source index and destination index (like MOVS) will increase both of them. In case it is set to 1 (using the set-direction-flag instruction STD), the instruction will decrease them.
This flag is used to determine the direction (forward or backward) in which several bytes of data will be copied from one place in the memory, to another. The direction is important mainly when the original data position in memory and the target data position overlap.
- 3.8.1 Carry and Direction Flag Control Instructions
- Direction Flag at MSDN
- Clear Direction Flag
- Set Direction Flag