INT 14 - MultiDOS Plus - TRANSMIT CHARACTER
        AH = 21h
        AL = character to send
        DX = port number
Return: AH = status (see below)
Note:   monitor mode must have been turned on with AH=24h before calling
SeeAlso: AH=20h"MultiDOS",AH=22h"MultiDOS",AH=24h"MultiDOS"

Values for status:
 00h    successful
 39h    no DSR or CTS
 3Ch    no DSR
 3Bh    no CTS
 41h    no such port
 42h    monitor mode not active
 97h    timed out

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