Technical information

NewBIOS Reference Serial Solutions
Service 3H simply reads the line status and modem status
registers of the 16450.
Other Points.____________
This service may prove useful to save a program from
waiting for the send or receive service to time-out. The user can
check the handshake and the receive/transmit buffers using
service 3H- if the port is not ready the user program can do
something useful while it waits for the port to get ready. When
ready use service 1H and/or 2H, knowing that they will return
immediately.
Chapter 5 Page 83