A program can open an auxiliary device for reading and writing by using Open File with Handle (Interrupt 21h Function 3Dh), supplying the device name AUX, and specifying the read-and-write access parameter. The function returns a file handle that the program can use with Read File or Device (Interrupt 21h Function 3Fh) and Write File or Device (Interrupt 21h Function 40h).
If the auxiliary device is not present or not ready to receive or send data, a program that reads or writes to the device may hold indefinitely. Before attempting to read from the auxiliary device, a program should use Check Device Input Status (Interrupt 21h Function 4406h) to determine if the device is present and ready to send input. Similarly, a program should use Check Device Output Status (Interrupt 21h Function 4407h) before attempting to send data to the auxiliary device.