The Walnut Kernel


The operating system component of the `Secure RISC Multiprocessor Project' is unofficially known as the `Walnut Kernel'. The aim of this project was to develop a capability based operating system with a simple model of system resources.

A capability is essentially a ticket which allows access to an object. The Walnut Kernel uses a password capability scheme. In practice this means that the Walnut Kernel's capabilities are 128 bits long and composed of an object name and a password. System security is derived from the sparse population of the name space. This means that it is necessary to guess the name of an object before it can be used. The low density of valid names ensures that it is necessary to guess many times to have a reasonable chance to access an object without explicitly being given a capability to the object.

The Walnut Kernel models all the system's resources as pages of memory. By adopting this paradigm it is possible to use the capability mechanism to provide access control to all the system resources.

The Walnut Kernel provides a simple uniform model of system resources and system security.

Parent Project

The `Secure RISC Multiprocessor Project' was tasked with developing a scalable multiprocessor system. The switching mechanism was required to scale with the addition of processor modules to the system. This contrasts with many existing systems where the switching infrastructure size is fixed and must be replaced to expand beyond its design dimension.

Dr Ronald Pose, and Maurice Castro developed the basic design of the scalable switching infrastructure.

Project Group

The Walnut Kernel was developed by Prof. Chris Wallace and Maurice Castro in the Department of Computer Science, Monash University. Mr Carlo Kopp and Mr Glen Pringle have developed libraries and environments for use under the kernel. Dr Ronald Pose and a number of his honours students have performed subsequent development.

Selected Relevant Publications

Castro, Maurice D. and Pose, Ronald D., Monash secure RISC multiprocessor: Multiple processors without a global clock, Australian Computer Science Communications, Proceedings of the Seventeenth Annual Computer Science Conference (ACSC-17) Christchurch, New Zealand, Editor Gupta G, pp 453-459, 1994

Castro, Maurice, The Walnut Kernel: User level programmer's guide, Technical Report 95/222, Department of Computer Science, Monash University, revised November 1995, 5, 1995 ( (userman.pdf)

Castro, Maurice and Pringle, Glen and Wallace, Chris, The Walnut Kernel: Program Implementation Under the Walnut Kernel, Technical Report, 95/230, Department of Computer Science, Monash University, Also released by SERC, CITRI as Technical Report SERC-0011, 8, 1995 ( (prgwal.pdf)

Castro, Maurice, The Walnut Kernel: A Password-Capability Based Operating System, Phd Thesis, 1996 ( (thesis.pdf)

The Name

The rational behind the name "Walnut Kernel" was both the obvious play on words and an in-joke amongst the developers; Walnut is a contraction of WALlace's Non UTopian as everything in the system costs money.