You have just designed the core of a retro computer. Your FPGA ULA will be faster, cooler, and more reliable than the original—but it will emulate the limitations perfectly (including the color clash, because that is the "flavor" of the machine).
It handles the complex "contention" between the Z80 CPU and the ULA. When the Z80 attempts to access lower RAM (0x4000-0x7FFF)—where the video data is stored—the ULA takes priority to ensure a stable picture.
Each revision brought small improvements in timing, power consumption and reliability. The existence of multiple ULA types also created challenges for software developers, who had to ensure their games worked correctly on all versions — a problem that is still felt today by retro‑computing enthusiasts. You have just designed the core of a retro computer
Use a ULA to consolidate peripheral logic (video, keyboard, memory control).
Many Spectrum games, especially those written in machine code, exploit this predictable contention pattern. Developers learned to place critical code and data outside the contended region (addresses above 0x8000) to achieve maximum performance, while using the contended region only for non‑time‑sensitive data. When the Z80 attempts to access lower RAM
The ZX Spectrum is a milestone in home computing history. Released in 1982 by Sinclair Research, this affordable machine brought computing into millions of European homes. At the absolute center of its elegant, low-cost design was a single custom chip: the Uncommitted Logic Array, or ULA.
This design choice is a perfect example of "ZX Design philosophy"—prioritizing cost and cleverness over raw, brute-force power. Use a ULA to consolidate peripheral logic (video,
Here’s the block diagram for your retro computer:
In an era of Raspberry Pi 5s running at 2.4GHz with 8GB of RAM, why learn how to design a machine with 16KB and a glue logic chip?
Instead of using an expensive dedicated keyboard controller chip, the ULA handled the 40-key rubber chiclet keyboard directly. It monitored 8 address lines and 5 data lines, passing raw matrix readouts directly to the Z80 CPU whenever an Input/Output (I/O) request was made. 4. Cassette and Audio I/O
The ZX Spectrum ULA (typically the Ferranti 5C112) is far more than just a simple glue-logic chip. It is responsible for the fundamental operation of the computer. A. Video Generation (The Display Engine)