Systems and methods are disclosed to support Host Initiated Processing (HIP), in which an application in a first system calls a software object in a second system that is in some respect incompatible with the first system. The calling application resides in a legacy system (e.g., an IBM mainframe), and is written based on a programming model (e.g., CICS). The called software object is based on a WINDOWS-based programming model (e.g., COM or .NET), which is incompatible with the legacy system's programming model. The HIP system converts a call from the legacy system so that the call is usable with the WINDOWS-based system, and also converts the result provided by the WINDOWS-based system into a form usable by the legacy system. Example aspects of incompatibilities between a legacy system and a WINDOWS-based system include: data representation, data types, communications protocol, passing of call parameters and return values, and error handling.