Direction flag
The direction flag is a flag that controls the left-to-right or right-to-left direction of string processing, stored in the FLAGS register on all x86-compatible CPUs. It is bit number 10.
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.
- If it is set to 0 (using the clear-direction-flag instruction CLD), it means that string is processed beginning from lowest to highest address; such instructions mode is called auto-incrementing mode. Both the source index and destination index (like MOVS) will increase them;
- In case it is set to 1 (using the set-direction-flag instruction STD), the string is processed from highest to lowest address. This is called auto-decrementing mode.
| x86-instruction | Meaning | Flag | Notes | |
|---|---|---|---|---|
| Direction
of string processing |
Mode title | |||
| CLD | clear direction flag | 0 | lowest-to-highest address | auto-incrementing |
| STD | set direction flag | 1 | highest-to-lowest address | auto-decrementing |
Sources and credits
This article is adapted from the Wikipedia article “Direction flag”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
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