Mechanisms for executing a software routine in an application executing as a multi-user single address space subsystem in an operating environment having a trusted mode of operation for trusted routines and a reduced-trust mode of operation for untrusted routines. The application includes a control module for execution as a trusted routine and a trusted routine table including identifiers of trusted routines. The control module performs switches between a trusted mode of operation for execution and a reduced trust mode of operation based on various determinations regarding the nature of a calling routine being trusted or untrusted, a call stack, and whether the calling routine is being restored or not from the call stack.