Using asynchronous exit routines

An authorized caller can request an asynchronous exit routine to run on behalf of a specific task.

Before the asynchronous exit routine runs, the calling program must initialize an interrupt request block (IRB) to identify the exit to the system and then schedule the asynchronous exit to run. The system then completes processing to get the asynchronous exit ready to run. See System processing to run an asynchronous exit.

There are several macros you can use to initialize the IRB and schedule the exit routine:

Initialize an IRB - The SCHEDIRB macro is the recommended interface for initializing an IRB for an asynchronous exit routine. See Using the SCHEDIRB macro to initialize and schedule an IRB.
You can also use the CIRB macro to initialize IRBs. This method allows you to reuse IRBs. See Using the CIRB macro to initialize an IRB.
Schedule the asynchronous exit - The SCHEDIRB macro is the IBM® recommended interface for scheduling an asynchronous exit. Using SCHEDIRB both to initialize and to schedule the IRB allows you more control over when your exit runs.
If you initialized the IRB for the exit using the CIRB macro, you can also use the SCHEDXIT macro to schedule your asynchronous exit, see Using the SCHEDXIT macro to schedule an IRB.

Asynchronous exits (IRBs) must save and restore all the Floating Point Registers and the Floating Point Control register if they use them. If an asynchronous exit uses Binary Floating Point (BFP) instructions, it can cause the IEEE exception flags and DXC in the Floating Point Control (FPC) register to change at unpredictable times relative to the main program. Therefore, asynchronous exits that use BFP instructions must save and restore the entire Floating Point Control register.

Note: Asynchronous exits that use decimal arithmetic instructions can cause the DXC to be set to zero if they take a data exception and the MVS™ task is enabled to save and restore the Additional Floating Point and Floating Point Control registers. In this case the asynchronous exit may, but is not required to, save and restore the Floating Point Control register. (The main program should not be affected by the DXC being set to zero because the DXC is normally used only when handling a data exception.)