The parameter area is set up by the mainline routine and passed to the recovery routine. See Setting up, passing, and accessing the parameter area.
The SDWA provides information to the recovery routine, and the recovery routine can communicate with the system, and with subsequent recovery routines, by placing information into the SDWA. See Using the SDWA.
When a recovery routine gets control, GPR 0 indicates whether an SDWA is available. When an SDWA is not available, the recovery routine can communicate its recovery options to the system only through registers. Aside from this circumstance, the recovery routine cannot use registers to communicate with the system; the routine must use the SDWA. Also, the mainline routine should not place information in registers and expect that information to be in the registers when the recovery routine gets control. Complete details about registers are in Understanding the recovery environment.