 EXCP's front-end is always in TCB mode, as EXCP is a Type 1 SVC.In MVS/370 and subsequent instances of the OS, the EXCP processor invokes STARTIO to schedule execution of the channel program.Although the EXCP processor's back-end is always in SRB mode, the back-end contains emulation code which allows the appendages developed for earlier, pre-MVS instances of the OS, specifically for EXCP, to function largely as before and thereby to appear to be in TCB mode, for which these appendages were originally designed.