Formal specification using mathematical text provides a sound basis for developing complex systems, especially safety-critical systems. The paper discusses three formal specification styles for state-based systems from monolithic, through Z to modular each illustrated by the same simple example. In each case, formal text which is left implicit in practice is identified and discussed. The paper includes a denotational semantics for operations within a modular system, including their conjunctive, parallel, choice and sequential composition. The importance of extending the semantics to unwritten formal text implicit in Z and modular style specifications is emphasised because misinterpretation of the implicit can have serious consequences.

When modelling a large and complex system, clarity of the specification becomes an important factor. In object-oriented specification, the states of individual objects are captured by the values of their attributes. Frequently however, there are dependencies between the attributes of an object. An appropriate indication of which attributes are primary (independent) and which are secondary (dependent) can add significantly to clarity. This paper details the notion of secondary attributes, their roles and implications in formal object-oriented specification.

Abstract This paper presents a systematic approach to formatted language design that incorporates formatting within the syntax of programming languages. The approach includes: a metasyntax to ensure that program text is foldable, not only to avoid right margin overflow but also to preclude constructs which are visually confusing or ambiguous; a set of guidelines for language designers to enhance readability within the constraints of the metasyntax; and a folding algorithm which selectively folds a program text. The resulting automatic formatting is consistent with current practice in program and text layout. The effect of this approach is to put program format decisions in the domain of the language's designer, rather than its several implementors or numerous users, which implies uniformly formatted programs of improved readability and therefore usability.

The paper describes a proposed microprogrammed interface computer, ``Intergraphic,'' which will link many (initially 13; potentially more than 50) general-purpose graphical terminals to a central processor. Intergraphic will generate new images once only, at high speed (10-MHz incremental plotting rates) on one of several small, electrostatically deflected, precision CRT's. The images will be generated on a 1024 by 1024 grid in incremental and random-point display modes by fast microcode sequences which interpret display lists from the central processor. The centrally generated images will be scan converted to standard television video signals and recorded on a multitrack video disk(s), each track refreshing a low-cost standard television terminal. User feedback will be via raster coordinates, determined by a light-pen (``raster-pen'') and simple counting circuits at each terminal. The paper concentrates on the digital interface structure which is versatile and fast (3-5 ns integrated circuits and 100-ns cycle-time READ-only memory). The description centers on display generation, although the structure is largely general purpose. The proposed operating system is outlined only since it is in an early stage of development. Also, the order-code accompanying data from the central processor is incomplete. However, new orders can be readily interpreted by adding microprograms to the READ-only memory (i.e., ``firmware,'' not hardware extensions); thus, the interface will also be a useful medium for experimentation in graphical structures and communication.

Techniques are presented which enable conventional "light-pen" tracking and pointing functions to be extended to the direct view storage tube (DVST). The schemes eliminate the high data transfer rates or considerable buffer storage required even for static displays when short persistence cathode ray tubes (CRTs) are used. Two solutions, a "quadruple photo-sensor pen" and the "potentiometer pen" (a transparent resistive sheet and stylus combination which covers the display), are described and compared.

The system design of a general-purpose digital computer, CIRRUS, is described. An extensive order code with reasonable order execution times has been achieved at relatively low cost. Comprehensive multiprogram operation is provided. These facilities have been produced largely with microprogramming. The basic hardware is comprised of general purpose registers, a half-word arithmetic unit and two stores, all of which may be inter-connected with considerable flexibility. A substantial contribution to economy has been made by the use of a prewired ``read-only'' store of large capacity for encoding microprograms.