Unless an ERP is in control, the system passes control to the SIO appendage just before the system translates and starts your channel program. It is called even if the channel program is not later translated. Your SIO appendages can build the channel program. The system does not test IOBSTART until after the SIO appendage returns. IOBSTART contains the virtual address of the start of the channel program.

The start I/O appendage may build a zHPF channel program or a CCW channel program. The caller does not have to set the IOBEZHPF bit prior to issuing the EXCPVR request. If the start I/O appendage builds a zHPF channel program, it should set the IOBEZHPF bit if it is not already set, and reset the IOBEFMT1 bit. Otherwise, it should reset the IOBEZHPF bit.

If the device is not enabled for zHPF or does not have the necessary capabilities, the start I/O appendage should either build a CCW channel program, or post the request in error and return to EXCP indicating that the I/O operation should be skipped (return +4).