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  Service 01H (decimal 1) transmits one character out the serial port
  specified in DX. When you call service 01H, you place the character to be
  transmitted in AL. When service 01H returns, it reports the status of the
  communications port. If AH = 00H, then the service was successful.
  Otherwise bit 7 of AH indicates that an error occurred, and the other bits
  of AH report the type of error. These bits are outlined in the discussion
  of service 03H, the status service.

  The error report supplied through this service has one anomaly: Because
  bit 7 reports that an error has occurred, it is not available to indicate
  a time-out error (as the details in service 03H would suggest).
  Consequently, when this service or service 02H reports an error, the
  simplest and most reliable way to check the nature of the error is to use
  the complete status report given by service 03H, rather than the
  less-complete status code returned with the error through services 01H and
  02H.