Book review: “The ZX Spectrum ULA”
If you’re anything like me (and considering you’re sat here reading this article on this very site, there’s a chance you might be) then you probably have more than just a passing interest in how your beloved 80s computers actually work. You may wonder how they manage to turn mere electricity into a wondrous device that can perform complex calculations in seconds, paint animated colour images onto the screen, or perhaps do a bit of both and bring you a challenging, absorbing arcade game.
One way to start finding the answers is to grab a copy of the machine’s service manuals. These are generally readily available from the internet and will show you exactly how the machine is wired together and what chips it uses. Sure, such manuals are great for repairing old systems, showing you how everything is wired together and often providing you with a number of troubleshooting flow diagrams. However, this is generally where they stop and it’s usually the responsibility of the reader to furnish themselves with any additional material. In the case of standard, off-the-shelf components such as the Z80 processor or the General Instruments AY sound chip, there is plenty of documentation on the internet that can help furnish you with answers. Despite this, one area that is often sadly lacking is insight into how the custom chips of these machines work, and it’s this that Chris Smith’s The ZX Spectrum ULA addresses.
While the centrepoint of this book is undoubtedly a rich and detailed deconstruction of the ‘soul’ of the ZX Spectrum â€“ the ULA, the book also provides an insight into many other areas of microcomputer design in the late 1970s and 1980s. The opening chapters help you understand the industry situation back then, fabrication techniques which will later help you to realise why the ULA was designed in a certain way and why the computer design in general followed a very specific design pattern.
If you’ve ever spent hours pouring over the schematic diagrams of various service manuals (believe me, I have) then this book will answer many of the questions that you might have. For example, why are there multiplexers between the memory chips and processor bus? How is the clock signal generated? As you read through, you will discover that every design decision has been made from a combination of technical and cost factors and the solutions presented are often ingenious. I constantly found myself experiencing a semi-eureka moment as I finally understood the reason for the presence of some of the support chips, which has helped me improve my understanding of how to troubleshoot broken computers as well as how to ‘programatically’ get the best out of them.
Of course â€“ as its title suggests â€“ this book spends a over half of its 300 pages honing in on the design of ULA itself. In particular, there is an extremely detailed description of perhaps the most exciting aspect of a computer’s features: generation of the video display. Questions answered along the way include: Was the attribute clash a bug or clever feature? Why does the machine have a border? Why is the display smaller than other computers? All these and more are answered in the six or so chapters dedicated to video generation.
The video chapters are really one area where you will appreciate having some knowledge of analogue TV systems. The book does go some way to describe how analogue television works, but additional knowledge will no doubt help you enormously and without it, youÂ may find it difficult to grasp some of the more complex concepts. Moving on from the video aspect of the ULA, the latter chapters talk about memory access then IO, including the keyboard and cassette storage system. Even the primitive sound capabilities of the original 48k machine get a brief description and within a few lines you quickly understand the limitations of sound.
The penultimate chapter is perhaps one of the most interesting as it dedicates itself to examining hidden features and bugs of the ZX Spectrum computer. Among the items discussed are why are there dark edges on flashing characters, the phantom key issue and the video
snowing effect. These are all ‘features’ that are very familiar to ZX Spectrum owners and the book explains why they happen.
In conclusion, this book is an absorbing, detailed and fascinating insight into the design and manufacture of the ZX Spectrum’s custom ULA. It is written in a style that is both technical and easy to understand, which is never an easy path to navigate, but this book does so with ease.The language isn’t overly complex making it truly accessible to all levels of technical knowledge. However, as can be expected of any book of this type â€“ the more you know about electronics, the more you’ll get out of it and the best bit is that as your knowledge grows, so too can your understanding of the concepts described. So rather than being a one-off read, this book will no doubtÂ become much-loved and long-term companion that you’ll find yourself referring back to again and again. Chris has clearly spent an enormous amount of time researching, understanding and explaining with passion how Sinclair made the ULA, and has generously shared this information. For die-hard Spectrum fans this book is an absolute must, as it reveals the innermost secrets of Sir Clive’s beloved microcomputer.
The book is published by ZX Design and Media (Chris’ own company) and is available for the bargain price of Â£23.50. Delivery is fast. Following my order, I received the book within three days which considering how near it was to Christmas is a remarkable achievement. I only hope now that Chris will go on to write many more books of this type. I for one would love to know more about the Amstrad CPC’s Gate Array or the custom chips of the C64. One thing is for sure, whatever Chris turns his hand to next â€“ I’m sure will be an excellent piece of work which will meet with great success!