but it is in at89c51rd2 = 256 bytes....
and in keil 128 ?????????

The "data" field of the linker summary is a mix of data and idata. It does mean the internal 256 bytes of RAM that an 8051 has.

You cannot directly recognize an overflow of data (0...127) space from this number. If such an overflow occurs, it will be indicated by a separate error message (or several).

Once again: You need to look at the linker listing files to get exact information on memory usage. The short summary given at the end of the linking process is meant as a rough overview only and lacks many details that only the listing file will provide.