Vagelis Hristidis (vagelis@cs.ucsd.edu)
Tue, 18 Apr 2000 01:44:49 -0700

Pilot: An operating System for a Personal Computer

Pilot was developed to be a single-user OS, so no emphasis was given to
security mechanisms. There is a strong coupling between it and Mesa
programming language, since Pilot was written in Mesa and can also be
thought of a runtime support package for Mesa. As Mesa does, Pilot also
emphasizes the distinction between an interface and an implementation.
The file system is flat, that means that no directory hierarchy is present.
Instead , a unique identifier of 64 bits is used to distinguish each file in
space and time. There is also the notion of Volume, which can correspond to
more than one physical storage device, and conversely a storage device can
have more than one logical Volumes. Virtual memory is implemented using a
tree hierarchy of spaces , which are contiguous runs of pages.
Direct access to files is allowed , since no strict protection mechanism is
In Pilot there is the notion of sockets as endpoints for communication
between machines.

Sharing and Protection in a Single Address Space OS

This paper describes the Opal Operating System .Opal is built on the base of
Mach 3.0 microkernel. It's main goal is t simplify sharing and protection of
information using a single address space model. Opal operates on 64-bit
architectures, so no addresses reuse is needed. An important aspect in Opal
is the separation of protection from addressing.
The execution context for threads is the protection domain, which restricts
their access to a specific set of segments. Segments are contiguous extents
of virtual pages.
A significant drawback of Opal compared to the addressing modes used today
is that we must use register relative addressing for private static data,
where registers take values dynamically, whereas in conventional systems
this data always has the same virtual addresses. Furthermore each resource
needs a capability to be accessed.