If you have a C compiler and most sprintf() routines are written in C, why the assembly language requirement? It can't be because of needing to use internal RAM only, since other compiler vendors' sprintf() routines can use only internal RAM. You probably only need to Google for a routine written in C (I found a bunch in about 15 seconds), then pick one that you think is most adaptable to suit your RAM and ROM requirements. Being more thorough with your search will provide some sprintf() routines that have some of the functionality removed (e.g., floating point conversions) an are thus less memory-hungry.