...call a function at an absolute address?

I want to call a function that is not part of my program and that is located at an absolute address, how can I do this?

In C/C++ code:

  typedef void (*fn_t)(unsigned long p1, unsigned long p2);

  fn_t fn = (fn_t)address;

  fn(10, 20);

In ARM assembly code:

  ldr r0, =10
  ldr r1, =20
  ldr r2, =address
  mov lr, pc
  bx r2