"After some research, I found that the c51s.lib had to be added.
I'm not sure of the reason why. Code written in "c" did not seem to need the c51s.lib."

When you compile a 'C' source file to .obj, the Linker "knows" that it came from a 'C' source, and automatically includes the necessary run-time support library; eg, c51s.lib.

If you have only one source file in your project, and use SRC, all the Linker sees is a single assembler-generated input file (it doesn't know that the assembler just happened to be generated by the 'C' compiler); therefore it assumes that this is a pure assembler Project, and does not automatically include the c51s.lib 'C' runtime support Library.

HTH - see the Keil site for more details.

This is one of those areas where uVision falls somewhat short of its "Total Integration" boast!
I prefer to say "NEIDE" - for Not Entirely Integrated Development Environment!