/*
     * Switch to stack of the new process and set up
     * his segmentation registers.
     */
    retu(rp->p_addr);
    sureg();
    /*
     * If the new process paused because it was
     * swapped out, set the stack level to the last call
     * to savu(u_ssav).  This means that the return
     * which is executed immediately after the call to aretu
     * actually returns from the last routine which did
     * the savu.
     *
     * You are not expected to understand this.
     */