Including BTS optional user-written exit routines

Saving registers and the save area chain

IMS Batch Terminal Simulator exit routines need to save registers in the save area that is pointed to by Register 13. This save area is provided at entry. Before processing any data, the exit routine must:

1. Get the forward save area
2. Set the save area address at offset 4 of the forward save area
3. Set the forward save area address at offset 8 of the save area
4. If necessary, make Register 13 point to the forward save area

Before returning control to IMS Batch Terminal Simulator, the routine must step back to the original save area and restore IMS Batch Terminal Simulator registers.