Kevin C. Kahn

A study of program locality and lifetime functions

A program model can be regarded as decomposible into two main parts. The macromodel captures the phase-transition behavior by specifying locality sets and their associated reference intervals (phases). The micromodel captures the reference patterns within phases. A semi-Markov model can be used at the macro level, while one of the simple early models (such as the random-reference or LRU stack) can be used at the micro level. This paper shows that, even in simplest form, this type of model is capable of reproducing known properties of empirical lifetime functions. A micromodel, alone without a macromodel, is incapable of doing so.

Supporting ada memory management in the iAPX-432

In this paper, we describe how the memory management mechanisms of the Intel iAPX-432 are used to implement the visibility rules of Ada. At any point in the execution of an Ada® program on the 432, the program has a protected address space that corresponds exactly to the programs accessibility at the corresponding point in the programs source. This close match of architecture and language did not occur because the 432 was designed to execute Ada—it was not. Rather, both Ada and the 432 are the result of very similar design goals. To illustrate this point, we compare, in their support for Ada, the memory management mechanisms of the 432 to those of traditional computers. The most notable differences occur in heap-space management and multitasking. With respect to the former, we describe a degree of hardware/software cooperation that is not typical of other systems. In the latter area, we show how Adas view of sharing is the same as the 432, but differs totally from the sharing permitted by traditional systems. A description of these differences provide some insight into the problems of implementing an Ada compiler for a traditional architecture.

iMAX: A multiprocessor operating system for an object-based computer

The Intel iAPX 432 is an object-based microcomputer which, together with its operating system iMAX, provides a multiprocessor computer system designed around the ideas of data abstraction. iMAX is implemented in Ada and provides, through its interface and facilities, an Ada view of the 432 system. Of paramount concern in this system is the uniformity of approach among the architecture, the operating system, and the language. Some interesting aspects of both the external and internal views of iMAX are discussed to illustrate this uniform approach.

The iMAX-432 object filing system

iMAX is the operating system for Intels iAPX-432 computer system. The iAPX-4321 is an object-oriented multiprocessor architecture that supports capability-based addressing. The object filing system is that part of iMAX that implements a permanent reliable object store. In this paper we describe the key elements of the iMAX object filing system design. We first contrast the concept of an object filing system with that of a conventional file system. We then describe the iMAX design paying particular attention to five problems that other object filing designs have either solved inadequately or failed to address. Finally, we discuss an effect of object filing on the programming semantics of Ada.

An L=S criterion for optimal multiprogramming

Balancing interpagefault lifetime (L) against page swap time (S) has always been a performance criterion of great intuitive appeal. This paper shows that, under normal conditions, controlling the memory policy parameter to enforce the constraint L ≥ S, and allowing the multiprogramming load to rise as high as demand warrants without violating this constraint, will produce a load slightly higher than optimum. Equivalently, using the criterion L &equil; uS for some u slightly larger than 1 will approximate an optimal load. Using simulations, this criterion is compared with two others reported in the literature, namely the “knee criterion” (operate with L at the knee of the lifetime curve) and the 50% criterion (operate with the paging device at 50% utilization).The knee criterion produced optimal loads more often than the L&equil;S criterion, which in turn produced optimal loads more often than the 50% criterion. Since no practical implementation of the knee criterion is known, the L&equil;S criterion is the most attractive of the three.

A small-scale operating system foundation for microprocessor applications

Sound engineering methodology, which has long been valued in hardware design, has been slower to develop in software design. This paper uses a case study of a small real-time system to discuss software design philosophies, with particular emphasis on the abstract machine view of systems. It demonstrates how the currently popular software design axioms of generality and modularity can be used to produce a software system that meets severe space constraints while remaining relatively portable across a family of microcomputers. These sorts of constraints have often been used to justify ad hoc design approaches in the past. The results of the project suggest that the use of such techniques actually make the meeting of many constraints easier than would a less organized approach. In addition, the reliability and maintainability of the resultant product is likely to be better.