A TSR can use the Multiplex interrupt under DOS version 2.x with certain limitations. Under version 2.x, only DOS's print spooler PRINT, itself a TSR program, provides an Interrupt 2Fh service. The Interrupt 2Fh vector remains null until PRINT or another TSR is installed that sets up a multiplex handler.
Therefore, a TSR running under version 2.x must first check the existing Interrupt 2Fh vector before installing a multiplex handler. The TSR locates the current Interrupt 2Fh handler through Function 35h (Get Interrupt Vector). If the function returns a null vector, the TSR's handler will be last in the chain of Interrupt 2Fh handlers. The handler must terminate with an IRET instruction rather than pass control to a nonexistent routine.
PRINT in DOS version 2.x does not pass control on to the previous handler. If the user intends to run PRINT under version 2.x, the program must be installed before other TSRs that also handle Interrupt 2Fh. This places PRINT's multiplex handler last in the chain of handlers.