Chain of Subroutine Calls

Now let us look at an example where subroutines call other subroutines. A subroutine that might call another subroutine must push the return address it gets onto the stack. When it returns to its caller, it pops the stack to get the return address it should use.

In the picture, main is called by the OS. As soon as main gets control it pushes $ra onto the stack (step 1). main computes for a while and then calls subA. subA immediately pushes the contents of $ra onto the stack (step 2). The return address that subA will use when it returns control to main is now on the top of the stack.

Next subA calls subB which pushes the contents of $ra onto the stack (step 3). The return address that subB should use when it returns to its caller is now on the top of the stack.

Now subB calls subC (step 4). subC does not call any subroutine so $ra does not have to be saved. subC computes for a while, and then returns to its caller with a jr $ra instruction (step 5).

Now subB has control again. The return address it needs to use is at the top of the stack. subB returns to its caller by popping the stack into $ra and executing jr $ra (step 6).

Now subA has control again. The return address it needs to use is at the top of the stack. subA returns to its caller by popping the stack into $ra and executing jr $ra (step 7).

Finally, main has control. After a computing for a while it returns to the OS by popping the stack into $ra and using the jr $ra instruction (step 8).