A quick introduction to simple functions

The PowerPC ABI is fairly complex, and will be covered in much greater detail in the next article. However, for functions which do not themselves call any functions and follow a few easy rules, the PowerPC ABI provides a greatly simplified function-call mechanism.
In order to qualify for the simplified ABI, your function must obey the following rules:

• It must not call any other function.
• It may only modify registers 3 through 12.
• It may only modify condition register fields cr0, cr1, cr5, cr6, and cr7.
• It must not alter the link register, unless it restores it before calling blr to return.

When functions are called, parameters are sent in registers, starting with register 3 and going through register 10, depending on the number of parameters. When the function returns, the return value must be stored in register 3.
So let's rewrite our maximum value program as a function, and call it from C.
The parameters we should pass are the pointer to the array as the first parameter (register 3), and the size of the array as the second parameter (register 4). Then, the maximum value will be placed into register 3 for the return value.