This is toolchain-dependent, and often boils down to linker magic.
As I said, I don't Keil, but I'd start with reading the fine manual... ;-)


The traditional way is to cast a constant (the address) to a function pointer and then invoke it.

typedef void (* function_ptr)();

int main(void) {
  ((function_ptr)0x8000)();
}
 

Less common methods would be to write a dummy function and move it to the given position again using linker magic and then removing it from the resulting hex by post-processing; defining a symbol at a required address using linker magic (together with writing a C prototype for it) (*); and the simplest: using inline assembler.

JW

[EDIT] Keil's linker does not appear to support custom symbol creation, making (*) unusable