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  Service 04H (decimal 4) is available only in the PS/2 ROM BIOS. It expands
  the capabilities of service 00H to provide support for the PS/2's improved
  serial ports. If you compare service 04H with service 00H, you'll find
  that the four serial port initialization parameters passed in AL in
  service 00H are separated into four registers in service 04H (Figure
  12-5). Also, service 04H returns both modem and line status in register
  AX, exactly as service 03H does. Because service 04H has these expanded
  capabilities, you should generally use it instead of service 00H for PS/2
  serial port initialization.

                           BREAK (register AL)
  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      No break
  01H                      Break
  --------------------------------------------------------------------------

                           PARITY (register BH)
  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      None
  01H                      Odd
  02H                      Even
  03H                      Stick parity odd
  04H                      Stick parity even
  --------------------------------------------------------------------------

                           BAUD RATE (register CL)
  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      110 baud
  01H                      150 baud
  02H                      300 baud
  03H                      600 baud
  04H                      1200 baud
  05H                      2400 baud
  06H                      4800 baud
  07H                      9600 baud
  08H                      19,200 baud
  --------------------------------------------------------------------------

                           STOP BITS (register BL)
  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      One
  01H                      Two (for word length = 6, 7, or 8)
                           11/2 (for word length = 5)
  --------------------------------------------------------------------------

                           WORD LENGTH (register CH)
  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      5 bits
  01H                      6 bits
  02H                      7 bits
  03H                      8 bits
  --------------------------------------------------------------------------

  Figure 12-5.  Register values for serial port initialization with
  interrupt 14H, service 04H. (Register DX contains a serial port number
  between 0 and 3.)