The authors have been developing a programming system intended to be especially convenient for scientific computing. Its main features are variable precision (decimal) floating-point arithmetic and convenient exception handling. The software implementation of the system has evolved over a number of years, and a partial hardware implementation of the arithmetic itself was constructed and used during the early stages of the project. Based on this experience, the authors have developed a set of specifications for an arithmetic coprocessor to support such a system. These specifications are described. An outline of the language features and how they can be used is also provided, to help justify the particular choice of coprocessor specifications. The authors also indicate what other hardware features would be most helpful to the systems programmer, especially for implementation of the exception handling.>

This paper describes some simple programming language facilities for raising and handling exceptions, and demonstrates, with a number of examples, their effectiveness in a scientific computing environment. Only a small number of predefined exceptions need to be introduced into the language, but any number of user-defined exceptions can he introduced by the programmer. A key feature is that handlers are attached to operators that can raise exceptions (including user-defined functions and procedures, as well as built-in operators), rather than to expressions or statements. The programmer has a great deal of freedom in specifying what action is to be taken in case an exception arises. The implementation and the applications are mainly concerned with scientific computing, but we believe the facilities are applicable in other areas as well.

A new computer arithmetic is described. Closely related built-in functions are included. A user's point of view is taken, so that the emphasis is on what language features are available to a user. The main new feature is flexible precision control of decimal floating-point arithmetic. It is intended that the language facilities be sufficient for describing numerical processes one might want to implement, while at the same time being simple to use, and implementable in a reasonably efficient manner. Illustrative examples are based on experience with an existing software implementation.

This paper describes the design of an arithmetic unit called CADAC (clean arithmetic with decimal base and controlled precision). Programming language specifications for carrying out "ideal" floating-point arithmetic are described first. These specifications include detailed requirements for dynamic precision control and exception handling, along with both complex and interval arithmetic at the level of a programming language such as Fortran or PL/I.