add    $t0,$s5,$s3   # calculate an important sum
      jal    somesub       # call a subroutine
      nop                  # branch delay
      mul    $s4,$t0,$v1   # multiply the sum by the result

Simple Linkage Convention

Here is an example of a calling convention. This convention is very simple and is not suitable for a serious program. But it illustrates some ideas that will be used later on in more complex conventions. Let us call it the Simple Linkage Convention . You have already seen most of the rules of this convention:

1. A subroutine is called using jal (which puts the return address in $ra.)
2. A subroutine will NOT call another subroutine.
3. The subroutine returns to its caller using jr $ra.
4. Registers are used as follows:
   • $t0 - $t9 — The subroutine is free to change these registers.
   • $s0 - $s7 — The subroutine must not change these registers.
   • $a0 - $a3 — These registers contain arguments for the subroutine. The subroutine can change them.
   • $v0 - $v1 — These registers contain values returned from the subroutine.
5. The main routine returns control by using the exit service (service 10) of the SPIM exception handler.

Since a subroutine may not call another subroutine (in this Simple Linkage Convention) programs consist of a main routine that calls any number of subroutines. But the subroutines do not call other subroutines and always return directly to main.