Seven-Step Program: Simplest "Leaf Function" Example

A "leaf function" is one that doesn't call any other function. If you drew a tree illustrating function calls in the program, it would be a leaf node in the tree. If the leaf function does not change any saved registers, it does not need to save anything to the Stack.

Calling Program:

• First save the "unsaved" registers if we will still care about them after procedure returns
  ($t0, ..., $t9, $a0, ..., $a3).

  Where do they get saved?   the Stack (we'll talk about this in a minute)

Place parameters in a place where the procedure can access them.
• Place the parameters in the "argument" registers ($a0, ..., $a3)
• Transfer control to the procedure.
  • The Jump And Link instruction (jal ProcedureAddress) does 2 things:
    • Store return location (address of next instruction after jump) in $ra. (Happens automatically.)
    • Jump to the specified address.

Procedure:

Acquire the storage resources needed for the procedure.
• A simple leaf subfunction does not need to do anything special here if it isn't going to use/clobber $s0, ..., $s7.
• Perform the desired task.
• Place the result values in a place where the calling program can access them ($v0, $v1).
Release the storage resources needed for the procedure.
• A simple leaf subfunction does not need to do anything special here if it hasn't used/clobbered $s0, ..., $s7.
• Return control to the point of origin.
  • Jump to the address stored in register $ra   (jr $ra).
    NOTE: The jr instruction jumps to an address stored in a register.

Calling Program:

• Calling program must retrieve values from $v0, $v1.

	    # CODE STRUCTURE

caller:     ...

            # put arguments in $a0 - $a3
            jal leafFunc
            # after call, do stuff using value(s) in $v0, $v1

            ...
          
            # return to "super-caller" or op. sys.


leafFunc:   # body of subfunction  (assume arguments are in $a0 - $a3)
            # put return value(s) in $v0, $v1
            jr $ra