PowerPC Assembly: How to pass and return multiple arguments?

Question

I am learning PPC Assembly but am having a hard time understanding the following about sub routine calls:

What registers can be used to pass arguments to a sub routine? I think it is R3 to R12? How do you pass arguments when you run out of registers? Could I please see a code example of this.

When passing back return values, can you only use R3? From what I understand you can only use this register to return a single value. If you need to return more values then you return a pointer to those values? Could I possibly see a code example for this too.

Answer 1

If you're writing in assembly language, you can do whatever you want. It's only when you want to interact with some external code (maybe a library, maybe a system call) that you need to obey the calling convention. Some architectures have multiple different ABIs, so you have to be careful.

If I recall correctly, pretty much everybody uses the same calling convention for 32-bit PowerPC, so this Apple reference should have everything you need to know. As you mention, parameters are commonly passed in R3-R10 and the return value is put in R3. This tutorial seems to indicate the same.