FPGAs allow for enhancements like VGA/HDMI output, SD card storage for instant game loading, and even "ULAPlus" for expanded color palettes. No-ULA Design: Some hobbyists, featured on
To design a modern microcomputer, you must decide whether to use a physical ULA, an FPGA implementation, or a microcontroller-based emulation. The Evolution of ZX Design: From Desktop to Portable
Select Your ProcessorWhile the Zilog Z80 is still in production, many designers use the Z80 CMOS version for lower power consumption in portable builds. Alternatively, an ESP32 or Raspberry Pi Pico can emulate the Z80 instruction set with ease. FPGAs allow for enhancements like VGA/HDMI output, SD
This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later.
+-------------------------------------------------------------+ | FERRANTI ULA | | | | +------------------+ +----------------+ +-----------+ | | | Video Timing |-->| Memory Control |-->| Audio/I/O | | | | (NTSC/PAL Sync) | | (Contended RAM)| | (Ear/Mic) | | | +------------------+ +----------------+ +-----------+ | +-------------------------------------------------------------+ What is a ULA? Alternatively, an ESP32 or Raspberry Pi Pico can
: Emulate the entire Spectrum in software, using PIO (Programmable I/O) for pixel output.
Today, the ULA's legacy lives on in the vibrant hobbyist scene that surrounds the Spectrum. Whether you are studying The ZX Spectrum ULA: How to Design a Microcomputer to learn the fundamentals of 8-bit architecture, building a CPLD replacement to resurrect a dead machine, or designing your own portable Speccy with a Raspberry Pi Pico and a 3D‑printed case, you are participating in a tradition of hands-on, accessible computing that Sinclair started more than forty years ago. The rubber keyed wonder may be long out of production, but its heart—the ULA—beats on, in schematics, in silicon, and in the imagination of everyone who ever typed LOAD "" and pressed play on a cassette recorder. If you share with third parties, their policies apply
This shared access to memory created an infamous quirk: . Because the ULA and the Z80 could not read the same RAM simultaneously, the ULA had to pause the CPU during screen drawing. The first 16 KB of RAM (addresses 0x4000 to 0x7FFF)—containing both screen data and essential program information—was known as "contended memory". Whenever the Z80 tried to access this region during active display time, the ULA would stretch its clock cycle, effectively slowing the processor down. During horizontal or vertical flyback periods, however, contention vanished, giving the CPU brief moments of full-speed access. This timing dance is crucial to understand for anyone designing a modern hardware replica: successful emulation or replacement must replicate these precise timing relationships to maintain software compatibility.
: Beyond just documentation, the book acts as a case study for designing an 8-bit microcomputer, covering memory contention, video display generation, and timing.
The enduring lesson of the ZX Spectrum ULA is that hardware limitations breed immense creativity. By forcing developers to work within tight constraints—such as color clash and halted CPU cycles—the ULA shaped the unique look, sound, and feel of 1980s European computing culture.
This article dissects the ULA’s architecture and then applies those lessons to the practical challenge of building a handheld, battery-powered Spectrum-compatible machine.