UIMs are invoked in task mode and in problem state. A UIM must call only UIM services (UIM service routines and UIM macros), not system services, because system services are not yet available when the UIMs are called at IPL time.

Link-edit UIMs with AMODE(31) and RMODE(ANY). UIMs must not change to 24-bit addressing mode.

The same copy of a UIM is used throughout HCD processing. Thus, a UIM can store information within itself and retain this information for HCD processing.

UIMs must use the standard register save area conventions. The UIM must set register 13 to point to its own register save area before calling any UIM service routines or before issuing the CBDIPPDS or CBDIMSG macro.