This article delves deep into the ZX Spectrum's ULA, exploring its architecture, its ingenious "memory contention" system, the technical challenges it faced, and why it serves as the ultimate case study for retro computer design.
When the ULA is actively drawing the visible part of the screen, it takes priority. This article delves deep into the ZX Spectrum's
This write-up covers: historical context, functional responsibilities of the ULA, high‑level design considerations for a retro-style microcomputer using a ULA-like chip, simplified block diagrams and practical implementation notes for hobbyists recreating or learning from the design. Both the Z80 CPU and the ULA need
Both the Z80 CPU and the ULA need access to the lower 16KB of RAM. Because video generation cannot be interrupted without tearing the screen display, the ULA takes absolute priority. If the Z80 attempts to read or write to lower RAM while the ULA is drawing the visible screen area, the ULA physically stops the Z80's clock signal. This is known as "contention," which slows down execution speeds slightly but protects video integrity. 3. Keyboard Matrix Scanning This is known as "contention," which slows down