 
 
 
                           INT 14H: Serial Port I/O                           
 
 These functions provide access to the two RS-232 ports supported by BIOS.

 The base address of each of up to 4 serial ports is stored starting at
 0:0400, but the POST tests for and initializes only the first two serial
 ports.

 See Asynchronous Adapter Ports for a description of the hardware interface.

AH  Service
 
00H initialize communications port
    Input: DX = port number (0-1)
           AL = initialization parameter bit flags:
           76543210Ŀ
            baud rate parity stplength 
           
            ͼ  ͼ       word length
                                           10=7 bits; 11=8 bits
                                stop bits 0=1; 1=2;
                           parity code
                                             x0=none; 01=odd; 11=even
                  baud rate  000=110; 100=1200
                                                         001=150; 101=2400
                                                         010=300; 110=4800
                                                         011=600; 111=9600
   Output: AH = comm status (see below)

01H send a character out the selected RS-232 port
    Input: DX = port number (0-1)
           AL = character to send
   Output: AL is preserved.
           If bit 7 of AH is set, an error occurred and
              AH (bits 6-0) = comm line status (see AH below)

02H receive a character from the selected RS-232 port
    Input: DX = port number (0-1)
   Output: AL is character received
           AH is non-zero if an error occurred

03H get communications port status
    Input: DX = port number (0-1)
   Output: AX is communications port status
           AH = line-control status       AL = modem status
            
           bit 7: timeout                 bit 7: received line detect signal
           bit 6: trans shift reg empty   bit 6: ring indicator
           bit 5: trans holding reg empty bit 5: data set ready
           bit 4: break detect            bit 4: clear to send
           bit 3: framing error           bit 3: delta recv line signal detect
           bit 2: parity error            bit 2: trailing edge ring detector
           bit 1: overrun error           bit 1: delta data set ready
           bit 0: data ready status       bit 0: delta clear to send

 BIOS Data (addresses of RS-232 ports, time-out value)     ROM-BIOS Functions
