we create the pty on the fly when it is first opened.
If you run out of ptys now, just MAKEDEV some more.
This also demonstrate the use of dev_t->si_tty_tty and dev_t->si_drv1
in a device driver.
work in progress and has never booted a real machine. Initial
development and testing was done using SimOS (see
http://simos.stanford.edu for details). On the SimOS simulator, this
port successfully reaches single-user mode and has been tested with
loads as high as one copy of /bin/ls :-).
Obtained from: partly from NetBSD/alpha
