The EXCP macro instruction initiates channel program I/O operations. Whenever you want to execute one of your channel programs, issue EXCP.

The format of the EXCP macro is:

>>-+-------+--EXCP--iob_addr-----------------------------------><
   '-label-'                   

iob_addr—RX-type address, (2-12), or (1)

The address of the IOB of the channel program to be executed.

If your program is also supplying an IOBE, then set register 0 to the address of the IOBE, and set flag IOBCEF on in the IOB to indicate that the IOB extension is present. An IOBE is required for format-1 CCW channel programs.