They fit in the same part of the address space as DRAM would be, and
are coherent (because they are not cacheable).
They are currently limited to single cores without DRAM. We intend
to lift both restrictions, probably when we add support for
heterogeneous tiles.
This usually shouldn't be used in Tiles that are meant to be P&R'd once
and multiply instantiated, as their RTL would no longer be homogeneous.
However, it is useful for conditionalizing RTL generation for
heterogeneous tiles.
The previous approach used ex_reg_valid to determine whether to
source data from the FPU or RoCC. Thus, when the RoCC was not
present, it was still creating muxes. Using ex_cp_valid instead
gets rid of them.
