This is where the ZX Spectrum ULA shines. The ULA reads screen memory ($4000 to $5AFF) and generates a PAL-compliant composite video signal.
The ULA decoded I/O addresses. It listened to ports 0xFE (254 decimal). This is where the ZX Spectrum ULA shines
Pro-tip for retro design: The ULA mapped the keyboard so poorly that pressing multiple keys would ghost. This was a feature, not a bug—it saved 10 diodes. Memory access arbitration
In a typical 1980s computer, the CPU (Z80) handles everything. In the Spectrum, the ULA acts as the traffic cop and video generator. Its four main tasks: I/O and peripheral glue
Design Lesson: Centralize all timing-critical functions into one programmable logic block. This reduces board complexity and component count.