Pictorial Summary

The picture shows main calling mySub. Two arguments are passed in $a0 and $a1. The subroutine uses the arguments in those registers.

In the picture, the arguments are set up with move and li instructions, but any means of loading the argument registers can be used.

The Simple Linkage Convention is limited in some obvious ways. A more advanced calling convention is discussed in the next chapter.

The caller passes arguments to the subroutine by placing them in registers. This is the only way that data is passed to the subroutine. The subroutine returns values to the caller in registers. This is the only way the subroutine returns data to the caller. The subroutine should not look at any memory used by the caller, and the caller should not look at any memory used by the subroutine.