ZX Spectrum ULA: How to Design a Microcomputer , written by Chris Smith, is considered the definitive technical resource for understanding the custom "heart" of the Sinclair ZX Spectrum. The book documents Smith’s extensive reverse-engineering project, where he stripped the ULA chip down to its transistors to reveal its hidden logic and design secrets. Core Technical Focus

Altwasser stared at the board. To fix it in hardware would require redesigning the chip, costing months they didn't have. "We don't change the hardware," he said, his voice steady. "We adapt the software."

However, the ULA was a double-edged sword. It was uncommitted. Once you printed the mask, you couldn't change it. The Spectrum’s infamous "attribute clash" (color fringing) wasn't a bug; it was a physical limitation of how many gates Altwasser could fit into the die to decode video memory.

Chapter 8: Testing & Debugging

Without a working ULA, debugging is hard. Build incrementally:

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