The ZX Spectrum ULA: How to Design a Microcomputer
With my ZX Spectrum Next due to ship any day now, I’ve been refreshing my memory on the clever creation of the original Sinclair ZX Spectrum, courtesy of Chris Smith’s book, The ZX Spectrum ULA: How to Design a Microcomputer.
It’s not a coffee table book. You’ll find no colourful glamour shots of microcomputers, or simulated screengrabs with clever CRTemulating free aliasing and scanlines. Instead, you’ll find an in-depth technical look at the heart of the Spectrum. Incredibly, Smith hadn’t even hit his teens when the Spectrum launched, and was in no way involved in its creation. With only a little help from original engineer Richard Altwasser and others in the Spectrum community, he’s worked to reverse engineer the device from first principles.
For anyone unfamiliar with the ULA, it’s effectively the secret sauce of how Sir Clive Sinclair was able to bring his 8-bit microcomputers to market at such a low cost. Designed by Scottish engineering firm Ferranti, the Uncommitted Logic Array (ULA) was a precursor to the modern field programmable gate array (FPGA) – an integrated circuit that could be modified to act in a particular way; in the case of the ULA, by permanently masking off particular sections.
A Ferranti ULA was behind the dramatic reduction in chip count between the ZX80 and the ZX81, and it’s no surprise to find one inside every ZX Spectrum. As Smith shows in his book, though, it’s the ULA that really makes the machine – far more so than the Spectrum’s Zilog Z80 processor or cheaper compatible, which could be readily found in any number of competing designs.
Beginning with a technical overview of integrated circuits (ICs), microcomputers, semi-custom ICs and the Ferranti ULA family itself, Smith offers a full breakdown of exactly what makes the Spectrum tick. The ZX Spectrum ULA is a treasure trove of hard-won technical information, from details of keyboard addressing and interrupts, through to the video display – an iconic system that stored bitmaps and their colours separately, allowing for greater detail in lower memory at the cost of colour clash, also known as attribute clash, where blocks of colour would follow a sprite around the playfield.
Even if your eyes glaze while attempting to follow the mathematical proofs contained in Appendix D, covering quadrature amplitude modulation, there’s plenty of interest for the Spectrum fan. For example, there’s a chapter on hidden features and errors, covering ‘phantom’ keys and the snow effect, a contention issue that causes parts of the screen to white out as if there were a snowstorm in your TV.
Smith deals with an extremely technical subject, and the result is a deeply technical book. If you’re looking for a trip down Nostalgia Boulevard, you’ll be better off with Chris Wilkins’ colourful The Story of the ZX Spectrum in Pixels or Bitmap Books’ Sinclair ZX Spectrum: A Visual Compendium.
But if you have lots of tech know-how or, like myself, fancy skimming through the pages and pretending to understand half of it, you won’t find a better or more detailed analysis. The ZX Spectrum ULA is available from www.
amazon.co.uk for £23.18 (VAT exempt).
There aren’t any shiny colour plates, but the diagrams are clear and informative
You’ll find no better reference to the Ferranti magic that made the ZX Spectrum possible