### A good answer might be:

Because there is noplace to put the return address. The register `$ra` is already in use.

# Pictoral Summary

The picture shows `main` calling `mySub`. Two arguments are passed, in `$a0` and `$a1`. The subroutine reads the arguments from 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 and the subroutine should communicate only through arguments and return values all contained in registers. The code for the subroutine should not use any symbolic address defined in the caller.

### QUESTION 10:

Should a code module know about the inner workings of another module?