The Vivado tools track two different types of placement:
- Fixed elements (shown in orange) are placed by you.
- Vivado stores fixed logic in the XDC.
- Fixed logic usually has a LOC constraint and can include a BEL constraint.
- Unfixed elements (shown in blue) placed by the tool.
In the following figure, the I/O and block RAM placement is fixed. The slice logic is unfixed.
Figure 1. Fixed and Unfixed Placement
