DOS Fn 50H: Set PSP

                                            Compatibility: 2.0+ (doc'd: 5.0)
 Expects: AH    50H
          BX    Process ID (i.e, PSP segment) to make current
 Returns:       none   
    Info: This function tells DOS that the PID▲ in BX identifies the
          current active process.  It is used in TSR programs to inform DOS
          which process is running; i.e., after popping up.

          Several DOS fns (including file handle operations and memory
          allocation functions), refer to and make changes to the PSP of
          the current process.

          A TSR should call Fn 51H and save its PSP during initialization.
          Before popping up, the TSR should use fn 51H to get the ID of the
          interrupted program, then use fn 50H to set the PID to the saved
          value.  Be sure to change the PID back before popping down.

See Also: PSP (Program Segment Prefix)
          Process Control Functions
          Program Startup & Exit
          DOS Environment
          DOS Functions