Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Int 21h Function 04h
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     If the output device is busy, this function waits until the device
     is ready to accept a character.
 
     There is no way to poll the status of the auxiliary device using
     this function. On the IBM PC, more precise control can be obtained
     by calling ROM BIOS Int 14h or by driving the communications
     controller directly.
 
     If CTRL+C is detected at the keyboard, an Int 23h is executed.
 
     You can also send strings to the auxiliary device by performing a
     write (Int 21h Function 40h) using the predefined handle for the
     standard auxiliary device (0003h) or using a handle obtained by
     opening the logical device AUX.
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