INT 14 - Digiboard DigiCHANNEL PC/X* - GET TRANSMIT BUFFER FREE SPACE
AH = 12h
DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)
Return: AX = number of bytes free

See also: 140A

1412 - Digiboard DigiCHANNEL PC/X* - GET TRANSMIT BUFFER FREE SPACE