The mbcl driver is a special serial line driver
which controls the processing and buffering of characters
for asynchronous serial lines. It is used by PPP
and SLIP to transmit and receive IP
datagrams over serial lines. The mbcl driver
copies the characters to be output from STREAMS
message blocks to clist buffers, then outputs
characters via the transmit buffer to the device. When the
serial line device hardware receives a character, the
mbcl driver interrupt function is entered. It
transfers characters from the device to a STREAMS
message block and puts the message upstream.