WHICH Delphi RTL? The only versions that I can compile with FPC without change (except for some minor bug fixes in the delphi code) are D3,4,5,6,7,2005,2006,2007. Others are possible too, but apart from XE2 (which won't work) I have no further licenses with full sourcecode, so can't test that. Like FPC, runtime libraries are highly version dependent.
But for the versions I mentioned it is possible to compile the full Delphi rtl with a FPC compiler, with the exception of system.pas.
The changes are:
- handles.
- parsing errors because of missing ';'
That's all.