INT 14 - ARTICOM - MODIFY FLOW CONTROL
        AX = 800Ah
        BL = flow control type (00h none, 01h XON/XOFF, 02h RTS/CTS)
        DX = port index (COM1=0, COM2=1, ...)
Return: AX = error code, if error (see AX=8003h)
Note:   for attached ports only!
SeeAlso: AX=8000h,AX=8003h,AX=8009h

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