SSI function code 50 is almost identical to SSI function code 4 (End-of-Task call). The only difference is that, for SSI function code 50, your function routine is given control before most resource managers are given control, whereas, for SSI function code 4, your function routine is given control after most resource managers are given control. If you are interested in obtaining control after most resource managers have been invoked, see SSI function code 4 (End-of Task).