How to implement " char * strcopy(char * DestStr, char * SourceStr, uchar StartPos, uchar Count

"Keil C51 is very good." ===> "C51 is v" strcopy(DestStr, SourceStr, 6, 8) ? How to implement it ?
How to implement " char * strcopy(char * DestStr, char * SourceStr, uchar StartPos, uchar Count

"Keil C51 is very good." ===> "C51 is v" strcopy(DestStr, SourceStr, 6, 8) ? How to implement it ?