Technical information

Serial Solutions NewBIOS Reference
Return Values._____________
AH is 16450 Line status, where
bit 7 = 0, always
bit 6 = Transmitter shift register empty
bit 5 = Transmitter holding register empty
bit 4 = Break interrupt
bit 3 = Framing error
bit 2 = Parity error
bit 1 = Overrun error
bit 0 = Data ready
AL is 16450 modem status, where
bit 7 = DCD, Data Carrier Detect
bit 6 = RI, Ring Indicator
bit 5 = DSR, Data Set Ready
bit 4 = CTS, Clear To Send
bit 3 = Delta DCD, set true if bit 7 has
changed since last read
bit 2 = Delta RI " " " " 6 " " " " "
bit 1 = Delta DSR " " " " 5 " " " " "
bit 0 = Delta CTS " " " " 4 " " " " "
These return values for AL, AH are the same as for
services 3H, 4H and 5H (they are generated by the same piece
of code).
Meaning._________
Service 0H, initialise, sets the parameters in AL for the
serial port specified by DX. Bits 7, 6, 5 are used to generate a
baud rate divisor for the 16450. These bits are then set to zero
and the byte is written to the 16450 line control register.
The line status and modem status registers are read and
returned by the AH and AL registers respectively. Bits 6, 5, 0 of
the line status reflect whether data can be moved in or out of the
16450, and bits 4, 3, 2, 1 denote various error conditions on the
receive line. The modem status records the state of the various
input lines apart from received data (Rx) on the RS232 port.
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