The individual input devices are identified by values corresponding to powers of 2 (1, 2, 4, 8...). Combinations of input device are specified by adding together these device numbers. Input device number 1 must be implemented and must correspond to the keyboard (there must always be some way for the user to interact with DP4 - for example the application must be able to respond to error messages). Note that if you have not defined any input devices in DFSETUP, the OSDI will automatically use input device 1.
Other input devices may be implemented as required. The mnemonics INP_KEYBOARD, INP_MSR and so on are defined for input device numbers in DIP.H.
Much of the time an application may specify a zero device input mask. In this case the OSDI should enable the keyboard. However, the DP4 terminal manager will expect the input source to be returned as 0 if the device input mask is 0 and 1 if it is 1. It will discard input not from the correct source. This has been known to cause problems in the past.
Input is handled by four or five functions, which you may modify in your OSDI: