Understanding the means of communication

An important aspect of writing a recovery routine is understanding how the recovery routine communicates with the mainline routine, the retry routine, and the system. This information discusses the following means of communication:

Parameter area
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.
SDWA
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.
Registers
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.

You should understand that communications are handled differently depending on the following circumstances:

Whether your recovery routine received an SDWA
Whether the communication is with the recovery routine or with the retry routine.