The Zx Spectrum Ula- How To Design A Microcomputer -zx Design Retro Computer- Jun 2026

Using Verilog or VHDL, you must create a master clock module. For a PAL Spectrum clone, your clock will run at . Divide this by 4 to get the 3.5 MHz CPU clock .

A modern programmer can still feel this contention. If you write a tight loop to fill screen memory on a real Spectrum, you can measure the delay—a direct echo of the ULA's iron grip on the bus. Using Verilog or VHDL, you must create a master clock module

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. A modern programmer can still feel this contention

The ULA continuously reads data from the video memory area in the lower RAM (addresses 0x4000 to 0x7FFF ). It decodes pixel bytes and attribute bytes (which control foreground and background colors) and serializes them into video signals. It also generates the video border color dynamically. 2. RAM Contention Handling The existence of multiple ULA types also created

Monitoring the 40-key membrane keyboard for input. Audio Output: Producing the "beeper" sound.

To prevent data collisions, the ULA controlled the clock signal to the CPU. If the Z80 tried to read or write to the shared "contended" RAM while the ULA was drawing a visible line on the TV screen, the ULA literally stopped the CPU clock, forcing the processor to wait for a few microseconds. Designing this contention logic required absolute timing precision. 3. Keyboard Matrix Interfacing