A very rare part is the UX-1200 board from Symbolics.
This is a Lisp machine. It can be plugged into a Sun
VME-Bus Server. I even have a suitable operating system
(Genera 8.3) for it. The UX-1200 (same CPU as the standalone
XL-1200) was (is) the biggest and fastest Lisp-Machine ever
built by Symbolics, before they went bankrupt. But there are
rumours, that some developers still continue working.
The Lisp machine does have (ok, ok, a very primitive)
LISP as assembly language. All I/O is done by the hosting Sun.
The Sun needs special device drivers to be able to control
the Lisp machine.
Access to the disks and to the ethernet is done through the Sun.
Each Lisp machine (a Sun can host up to five) needs a "world",
which contains their files and programs. From the outside view
this is a huge (300 MB and more) file.
Genera is the name of Symbolics' operating system. The latest
version is 8.3. There are a lot of nifty programs available
for Genera, for example the Emacs (what else!?!), a C-compiler,
a Web-server and many many more. Especially the user interface
is funny: The Lisp machine simply opens its Lisp browser on
a given X-Terminal or X-Display, when the board is activated.
This is accomplished by running a program on the hosting Sun.