An application can issue mmioSendMessage to send private messages to an installable I/O procedure. This function enables a program to call an I/O procedure directly (unlike system messages, which should be sent by the MMIO Manager). MMIOOS2.H in the \TOOLKIT\H subdirectory defines the identifier MMIOM_USER so that you can create your own messages. The mmioSendMessage function requires that your custom messages be defined between the MMIOM_USER and MMIOM_USER_END values defined in the MMIOOS2.H file.