INT 14 - PC-MOS/386 v5.01 $serial.sys v5.04 - CHANGE PORT PROTOCOL
        AH = 05h
        AL = bit 7: set to enable/disable CD monitoring, clear to set protocol
             ---bit 7 set---
             bit 4: CD monitoring enabled
             bit 5: automatic restart enabled
             ---bit 7 clear---
             bit 0: receive XON/XOFF
             bit 1: transmit XON/XOFF
             bit 2: DTR/DSR
             bit 3: RTS/CTS
        BH = new XOFF character
        BL = new XON character
        DX = port number
Return: AH = FFh if invalid protocol
SeeAlso: AH=00h,AH=04h"SERIAL",AH=06h"PC-MOS"

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