The Flite 68000: An Introduction

Twenty years ago it didn't really matter which chip was selected for training purposes, as they all had very much the same characteristics. In fact   the first generation 8-bit microprocessors were suited to teaching better basic computer theory or simple interfacing techniques, than to illustrating the principles of modern computer architecture. Today, its not quite as easy to select a particular microprocessor as a vehicle for teaching. There are now many devices to choose from and the requirements of the syllabus have radically changed over the past twenty years. Topics once thought of as final year or even postgraduate level have been brought forward to the second year syllabus.

Teachers of computer architecture today want to use microprocessors to teach the fundamentals of operating systems, multitasking, memory management, real-time processing and bus protocols.

The device chosen by many universities, to illustrate a large part of the syllabus is the 68000 and its successors. The 68000 is so well-suited to an educational environment that if it had been designed purely as an educational tool, it could not have been improved, until recently, the lack of suitable 68000 based training systems at a reasonable price presented academics with a dilemma. In the absence of suitable hardware they could either teach the 68000's architecture on the chalkboard or as a paper exercise.

However, the equipment available today makes it possible to have the best of both worlds. The architecturally superior 68000 family can be used to support a variety of courses using a low cost single board computer.

The 68000 fits neatly into the teaching of computer science and electronics, its streamlined but powerful architecture makes it much easier to teach assembly programming than many of its predecessors arid contemporaries.

Not only is it relatively easy to start learning the 68000's assembly language but its more advanced features (sophisticated addressing modes and support for stack frames) also mean that the 68000 can be used to demonstrate advanced topics, ranging from data structure to the way in which the output of compilers is mapped on to machine code. All members of the 68000 family operate on one of two modes (user or supervisor state), thus it is remarkably easy to introduce the protection mechanisms required by operating systems. These protection mechanisms enable an operating system to guard against either accidental or malicious damage caused by a user program running under the operating system.

Other attributes of the 68000 include the multi-level prioritized, vectored interrupt system, synchronous bus interface and bus arbitration pins.

Access to Hardware

Students require hands-on experience to help them really appreciate how many of the 68000's facilities are actually used.

Although several 68000-based commercial computers are widely available, they are often little more than 'black boxes' as far as the user is concerned. It can be very difficult to access the 68000 processor at the heart of these machines and to perform all the required investigations.

Flite Electronics has designed a 68000-based single board computer that enables students to investigate the operation of these devices, both at hardware and software levels. A comprehensive monitor permits the student to enter a program in mnemonic form and to debug it with all the relevant tools - single instruction trace, breakpoints and memory dumps.

Terminals are not included but either a conventional dumb terminal or PC can be used. Suitable PC software and a connector enabling the PC to be interfaced to the microcomputer can be provided by Flite Electronics International Limted.

More importantly the cross-assembler running on the PC can be used to develop 68000 (or 68020) assembly language programs on the PC, for subsequent downloading onto the 68000 systems. Cross-assembly allows the student to use all the PC's development facilities (editor and file store).

In addition, to the 68000 assembler, a C cross-compiler is offered which enables students to design 68000 programs in a high level language. Students can develop programs in C to control interfaces, such as the PI/T and the 68631 dual universal asynchronous receiver transmitters (duart) on the single board computer.

Copyright © Written by Professor Alan Clements

Lecturer in Applied Computing Teeside University and one of the United Kingdoms leading authorities on the 68000 family. He acts as a consultant to both Motorola and Flite Electronics International Limited.