INT 14,1 - Send Character to Communications Port

AH = 01

AL = character to send

DX = zero based serial port number (0-1) (0-3 for AT)

on return:

AH = port status (see ~INT 14,STATUS~)

bit 7=0 indicates success

bit 7=1 indicates error, bits 0-6 indicate cause

- ~INT 14,3~ should be used to determine the actual cause

of the error since the time-out bit of the status register

is always set during an error on this call

- uses hardware flow control