I have no expertise with chip level details like preload engine. It looks like you are having trouble writing an assembly routine you call from C.
I presume you are using the TI ARM compiler. That means you are building according to the EABI application binary interface. That interface is standard for all ARM compilers. A web search on ARM EABI calling convention yields this document from ARM as the first result. That document contains all the details on writing an assembly routine that can be called by any ARM compiler, including the TI ARM compiler. The important detail is that the first argument is passed in R0.
Hope this helps ...
-George