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Check to see if the file is in the right place. It's likely that your directory structure is different, but it should be a subdirectory of keil. Simply use the generic code #include<8051.h> or whatever the generic code is. Look through the header files in the "include" directory. There are warnings for a reason! If you save "that" definition file in post 6 to your working directory as "REGX51.H," you'll be fine. Then you remove #include<8051.h> and replace it with #include"REGX51.H"
Now if only we knew which "post 6" that is you're referring to...