JCL: What is EXCP ?

EXCP stands for EXecute Channel Program. These are the I/O subsystem hardware driver programs that do the actual data transfer between the DASD (Hard disk) and the system memory(core/RAM). Channels in mainframes are similar to Buses in the PCs. They are basically the electrical paths to carry data. So each time a trip is made to the DASD to fetch data, EXCP count goes up by one. This is the reason we say a job is looping if the EXCP count is not increasing. Note that the count goes up by one for each block of data transferred. So a data transfer of a single 4K block and a single 32K block will count for one EXCP each.

There is a little caveat here. If the data being fetched is in a DB2 table, the EXCP count will NOT go up even though large amounts of data are being transferred. That is because the EXCPs are logged by a part of MVS called SMF (Systems Management Facility) while DB2 I/Os are handled by MMF (Media Manager Facility). So those I/Os don’t show up on the EXCP reports.