The number of I/O service units is a measurement of individual data set I/O activity and JES spool reads and writes for all data sets associated with an address space. SRM calculates I/O service using I/O block (EXCP) counts.

When an address space executes in cross-memory mode (that is, during either secondary addressing mode or a cross-memory call), the EXCP counts are included in the I/O service total. This I/O service is not counted for the address space that is the target of the cross-memory reference.