Acorn User

Guide To The BBC Rom
By Melbourne House
BBC Model B

Published in Acorn User #043

MOS And Basic Obscured

This 230 page book covers the ins and outs of the BBC Machine Operating System (OS 1.2) and the Basic ROM. In fact the majority of the book is given over to a discussion of the MOS.

The author breaks this down quite neatly into sensible segments. Chapters cover initialisation, interrupts, OSBYTE and OSWORD calls, input and output, buffers, the keyboard, VDU control, save and load, the sound system and files.

The approach taken is that of a partial descriptive disassembly. The address of the start of a block of instructions is given, followed by a description of what that block of code does.

It does not provide a disassembly listing in the traditional sense - frightened off, understandably, by Acorn's strong copyright stance - though you really would need to have one to make any sense of what's actually happening.

Unfortunately, the author has not put a lot of thought into the presentation of his material which is often difficult to understand. This is exemplified in the final chapter on Basic. Don Thomasson goes to great pains, early in the book, to point out that there are different versions of the MOS and Basic around. However, nowhere to my knowledge does he tell you which version of Basic he is discussing. There are four for the BBC to my knowledge, I know that he is talking about Basic 1, but would the average Basic 2 user (who would be baffled when the addresses listed throughout the book did not tie up)?

The chapter about Basic should not be regarded as a guide to the interpreter. Indeed, that would require a book to itself (such as the Basic ROM User Guide by Mark Plumbley). The information given here is more of a general overview.

If you are willing to produce a disassembly listing of the MOS, then this book will help you some way towards understanding what it does and how it does it, though you will need to sweat a bit.

Bruce Smith