Machine code is a difficult subject both to explain as well as learn. The Stephenson duo do their best to give useful comments in the assembler listings but for the newcomer this will not prove enough and here the clarity of the authors' explanation in the text is fully tested. In my opinion, they passed with flying colours. The authors themselves are not averse to admitting the often considerable difficulty in grasping the complexities of directly programming BBC Micro components, especially the 6522 VIA. This understanding leads them to fully describe the effects of each programming operation for programmers less well versed in the world of registers, flags, protocol and handshaking, all driven by the laws of binary logic which BASIC so effectively 'masks out'.

This book is not for the machine code beginner, but anyone who has dabbled will get enjoyment both from the hardware and software information given here and the assembler listings which demonstrate the sort of speed required in sorting large quantities of data and in moving chunks of colour graphics smoothly around the screen (the authors don't restrict themselves to Operating System calls but do demonstrate their use as well as direct screen addressing).

Chapters outlining the architecture of the Beeb and looking at the resident assembler make a good introduction before moving on to particular implementations of machine code. It is rightly emphasised that the best way to use the assembler is (just as Acorn designed their chip) from within BASIC.

Peter Mujtaba