Initiating the Channel Program

Issuing EXCP or EXCPVR macro requests execution of the channel program specified in the IOB. The system validates the request by checking fields of the control blocks associated with this request. If the system detects invalid information in a control block, it initiates abnormal termination procedures. The system gets the address of the:

DCB from the IOB
DEB from the DCB
UCB from the DEB.

If this is an EXCPVR request and you have provided a page-fix appendage, the system passes control to it to allow you to either page fix your channel program and data areas, or to provide EXCP with a page fix list so that EXCP will do the page fixing. For a description of the page-fix appendage and its linkage to the system, see Page Fix and EXCPVR Start I/O Appendage.

If you have provided a start I/O (SIO) appendage, the system passes control to it. The system does not examine the channel program until the return from the SIO appendage.The return address from the SIO appendage determines whether the system executes or skips the I/O operation. For a description of the SIO appendage and its linkage to the system, see Start-I/O Appendage.