Start-I/O Appendage
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).
- Reg. 14 + 0 — Normal return. The system should translate the channel program, if required, and initiate the I/O.
- Reg. 14 + 4 — Skip the I/O operation.
The channel program is not initiated. The channel program is not posted
complete. You can post the channel program complete by using the POST
macro, as follows.
- Set the completion code in register 10. This register is used to post the ECB.
- Set register 11 to the ECB address in the IOB.
- Issue the POST macro as shown in the example below:
For more information on the POST macro, see z/OS MVS Programming: Authorized Assembler Services Guide and z/OS MVS Programming: Authorized Assembler Services Reference LLA-SDUPOST (11),(10),LINKAGE=BRANCH