Technical information

NewBIOS Reference Serial Solutions
Service AFH, Get/Set IRQ line.
This service exists to support NewCOM, which can use
interrupts for more convenient and efficient serial
input/output. Subservice 0 returns the IRQ line which a
particular com port is assumed to be using, and subservice
1 sets this. Ports COM1 to COM8 are given default values
(which may need to be altered), but ports COM9 onwards
must be explicitly set before NewCOM can use them in
interrupt mode.
Service B0H, Get/Set Hardware Handshake.
This service exists to support NewCOM, which can use
several different hardware handshakes. The hardware
handshake is the way that the control lines associated with
a serial port are used. The input lines (CTS, DSR, DCD
and RI) are checked to ensure that it is ok to send data
out, and the output lines (RTS and DTR) are set to say
whether the driver has space to read any more data. The
handshake selected by this service defines which lines are
used, and how. This service also returns a flag to show
whether the driver, NewCOM, is present, as opposed to
NewBIOS alone.
Service B1H, Get/Set Software Handshake.
This service exists to support NewCOM, which can use
software handshakes. A software handshake uses special
characters, conventionally called XON and XOFF, to
control the flow of data. This is to be compared with a
hardware handshake, where control lines fulfil the same
purpose. Subservices 0 and 1 allow software handshaking
to be turned on and off, and subservices 2 and 3 allow the
actual characters to be used as the flow control characters,
XON and XOFF, to be changed.
Chapter 5 Page 73