Comparing EXCP and EXCPVR

EXCP and EXCPVR are two macros you can use to initiate channel program I/O operations, or as it is often put, execute a channel program. Both provide the same function - a device dependent way to perform I/O operations. However, there are a number of differences:

In order to issue an EXCPVR request, your program must run in a protection key between zero and seven, run in supervisor state, or be APF authorized. An EXCP request, on the other hand, can be issued by programs running in any key, including user key (key 8) or problem state.
If you issue an EXCPVR request, your program is responsible for translating its own virtual channel program into a real channel program. This includes page fixing your channel program and I/O buffers either before issuing the EXCPVR request, or by using the page fix appendage and updating your channel program with real addresses, building indirect address lists when needed, and updating the address fields within your channel program with real addresses. This allows your program to improve the efficiency of I/O operations in a paging environment, but does add some complexity to your program.
If you issue an EXCP request, you supply a virtual channel program. In other words, the channel program contains the virtual addresses of storage areas that may be in pageable storage, and the system is responsible for translating your virtual channel program into a real channel program.

Note that EXCP requests issued in an APF-authorized program in a V=R address space (EXCP V=R requests, in other words) are not translated. A V=R address space is one defined with ADDRSPC=REAL in the JCL EXEC statement. Because the address space is V=R, any CCWs created by the user already have correct real data addresses. (Translation would only re-create the user's channel program, so the CCWs are used directly.)

For information on using APF, see z/OS MVS Programming: Authorized Assembler Services Guide.

Table 1. Summary of the differences between EXCP, EXCPVR, and EXCP V=R
Function	EXCP Characteristics	EXCPVR Characteristics	EXCP V=R Characteristics
Program state and key	Any protection key, supervisor or problem state.	Protection keys 0-7, supervisor state, or APF authorized.	Protection keys 0-7, supervisor state, or APF authorized.
User responsible for translating their channel program	No	Yes	No translation required.
CCW formats supported	0 and 1	0 and 1	0 and 1
Virtual IDAW supported	Yes	No	No
MIDAWs supported	No	Yes	No
High Performance FICON® for System z® (zHPF) channel programs supported	Yes	Yes	No
User can modify channel program during execution	No	Yes, non-zHPF only	Yes
Self modifying channel programs supported	No	Yes, non-zHPF only	Yes