This tutorial paper explores the mechanics of protecting computer-stored information from unauthorized use or modification. It concentrates on those architectural structures-whether hardware or software-that are necessary to support information protection. The paper develops in three main sections. Section I describes desired functions, design principles, and examples of elementary protection and authentication mechanisms. Any reader familiar with computers should find the first section to be reasonably accessible. Section II requires some familiarity with descriptor-based computer architecture. It examines in depth the principles of modern protection architectures and the relation between capability systems and access control list systems, and ends with a brief analysts of protected subsystems and protected objects. The reader who is dismayed by either the prerequisites or the level of detail in the second section may wish to skip to Section III, which reviews the state of the art and current research projects and provides suggestions for further reading.

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The protection of information in computer systems

Various addressing schemes making use of segment tables are examined. The inadequacies of these schemes when dealing with shared addresses are explained. These inadequacies are traced to the lack of an efficient absolute address for objects in these systems. The direct use of a capability as an address is shown to overcome these difficulties because it provides the needed absolute address. Implementation of capability-based addressing is discussed. It is predicted that the use of tags to identify capabilities will dominate. A hardware address translation scheme which never requires the modification of the representation of capabilities is suggested. The scheme uses a main memory hash table for obtaining a segment's location in main memory given its unique code. The hash table is avoided for recently accessed segments by means of a set of associative registers. A computer using capability-based addressing may be substantially superior to present systems on the basis of protection, simplicity of programming conventions, and efficient implementation.

Capability-based addressing

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Modern Operating Systems

An object code expansion program inserts new instructions and data between preexisting instructions and data of an object code file; offsets are modified to reflect new positions of the preexisting instructions and data. For each item of preexisting object code (instructions or data), the following steps are performed: making a new code block comprising any desired new instructions and the item, and storing it as new object code; tracking the location of the item and the new code block within the new object code; and tracking items that contain inter-item offsets. Then, each inter-item offset is updated using the new location of the item or new code block, as required. Finally, offsets in symbol tables and relocation structures are updated with the new location of the item. This expansion program is used to add instructions to object code files of a second program, to monitor substantially all of the memory accesses of the second program. The added instructions establish and maintain a memory status array with entries for memory locations that are validly accessible by the second program; entries indicate the status of corresponding memory locations. The memory status array is used to check for the errors of writing to unallocated memory and reading from unallocated or uninitialized memory. Also, the data section of the object code files are expanded with extra dummy entries to aid in the detection of array bounds violations and similar data errors. Furthermore, watchpoints can be established for more comprehensive monitoring.

Method and apparatus for modifying relocatable object code files and monitoring programs

This paper develops four related architectural principles which can guide the construction of error-tolerant operating systems. The fundamental principle, system closure, specifies that no action is permissible unless explicitly authorized. The capability based machine is the most efficient known embodiment of this principle: it allows efficient small access domains, multiple domain processes without a privileged mode of operation, and user and system descriptor information protected by the same mechanism. System closure implies a second principle, resource control, that prevents processes from exchanging information via residual values left in physical resource units. These two principles enable a third, decision verification by failure-independent processes. These principles enable prompt error detection and cost-effective recovery. Implementations of these principles are given for process management, interrupts and traps, store access through capabilities, protected procedure entry, and tagged architecture.

Fault Tolerant Operating Systems

This paper proposes that all data elements in a computer memory be made to be self-identifying by means of a tag. The paper shows that the advantages of the change from the traditional von Neumann machine to tagged architecture are seen in all software areas including programming systems, operating systems, debugging systems, and systems of software instrumentation. It discusses the advantages that accrue to the hardware designer in the implementation and gives examples for large- and small-scale systems. The economic costs of such an implementation for a minicomputer system are examined. The paper concludes that such a machine architecture may well be a suitable replacement for the traditional von Neumann architecture.

On The Advantages of Tagged Architecture

In this paper we report on a new computer with several novel features. These features are applications of the concept of tagged architecture, and although some of them are not unique to the Rice Research Computer (R-2), they focus our attention on this radical design form, its advantages and disadvantages. Since the work is still in progress, we limit this report to a discussion of the architecture and a few of its ramifications.

The Rice research computer: a tagged architecture

The Rice Research Computer - A tagged architecture

This paper develops the concept of resource structuring in an attempt to remove the structural rigidity present in a conventional computer system. It is claimed that the concept can lead to building better and more efficient computer systems. A language is developed to assist in resource structuring. The language can specify structures such as an arithmetic pipeline or an array configuration. It handles vector and matrix applications efficiently and can be used at the user as well as the system level. Resource structuring in a computer system has other advantages which include reduction of supervisory control on operand routing and transformation of the system to exhibit failsoft behavior. To implement structuring, tagged architecture is suggested and shown to be an eligible candidate.

An approach to restructurable computer systems

This session will begin with presentations and a panel discussion by representatives of various governmental agencies involved with research or funding of research in data security. They will present their views on the policies and goals of federally funded research in the area of data security and how this relates to privately funded research and development in this and related areas. They will also have the opportunity to point out new directions which should be pursued. There will be discussion on both the technological and social impact of such funding.

E. A. Feustel

Papers

On The Advantages of Tagged Architecture

The Rice research computer: a tagged architecture

The Rice Research Computer - A tagged architecture

An approach to restructurable computer systems

Research in data security: policies and projects