For EXCP requests that run in a V=V address space, the system builds a translated copy of your channel program in real storage. Channel program changes made by your page-fix or start-I/O appendage, which run before the channel program is translated, will affect the real channel program. Later changes to your copy of your channel program with processor instructions or with data read in by an I/O operation will not affect the real translated channel program. Thus, in a V=V address space any attempt to modify an active channel program affects only the virtual image of the channel program, not the real channel program being executed by the channel subsystem. If you wish your changed channel program to be executed, your program can issue another EXCP macro or your channel-end appendage can return at offset 8.

Modifying a channel program during execution is supported for EXCPVR and EXCP requests that run in a V=R address space, since the system does not build a translated copy of your channel program. Modifying a channel program during execution is only supported for CCW channel programs, not zHPF channel programs.