title: How to define an ARM assembly function that can be used in C
body: <p>I've learnt C for few months already and now I'm starting to take a deeper look into the lower level language -  ARM assembly, so, I decided to start with a very basic project, making an assembly file .S that defines a factorial function and then calling and using it in C. So I want to ask whether assembly can be used to define such a complicate function and can be used as another functions in C? If you don't mind, please give me a simple example of .S file in which a function loads 2 numbers from standard input num1 and num2 to registers, computes num1*num2 and returns the result is defined and can be used in C source file?</p>
  
  <p>Thank you for reading. If this question is stupid, at least, please give me a direction so that I can follow and find out it by myself.</p>