"Keil C51 is very good." ===> "C51 is v" strcopy(DestStr, SourceStr, 6, 8) ? How to implement it ?
K&R shows implementations of some of the standard string library functions as examples. You should be able to work from there?
Or for even more examples: http://www.freebsd.org/cgi/cvsweb.cgi/src/lib/libc/string/