Opal and Pilot eval

John-Paul Fryckman (fryckman@SDSC.EDU)
Tue, 18 Apr 2000 04:23:12 -0700 (PDT)


Pilot's main propose exploits a system dedicated to a single
user. A lot of complexities rising from a multi-user system
are removed in Pilot. This is important because as the price
drops, it becomes economical for individuals to have their own
personal computers.

Its approach to a single user system is to limit features for
protection and resource allocation. They use the Mesa language
to implement their OS, Mesa provides the interface for various
system tasks. A defensive and not absolute posture is taken for
protection. Furthermore, capabilities are used to implement
protection for files. However, the system is vulnerable to malicious

The paper does a good job describing a system in which there
is only one user. There are many aspects that become simplified
or excluded in such a system. These points were noted and easily
read by me.


Opal's main design goal was to build an OS to take advantage
of a 64-bit addressing range leading to a single shared context-
independent address space. A major reason why context-dependent
address spaces were because the physical address space was limited
and it had to be shared amongst all the processes. With 64-bits,
it is no longer a problem and the over-head of having to manage
such limited addresses spaces can be done away with in a
wide-address architecture.

Therefore, the primary goal of this paper was to establish
the groundwork for a shared address space. The first benefit
is that a pointer reference is valid in all processes, and IPCs
become as simple as passing pointers. Protection is provided by
capabilities assigned to the various threads, essentially a
thread has access to every last segment of memory, however, the
capability assigned to the thread typically limits it scope.

The paper takes ideas from Multics, Hydra, Pilot as well as
many other OSes. Capabilities and ACLs are taken from Hydra
and Multics. And a single virtual address space was taken from
Pilot. Even though the paper was very long, it was clearly
written and throughly described the basics of Opal.