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

This is where the ZX Spectrum ULA shines. The ULA reads screen memory ($4000 to $5AFF) and generates a PAL-compliant composite video signal.

Before the era of FPGAs and cheap microcontrollers, there was the ULA. Think of it as a prefabricated silicon breadboard. Ferranti, the manufacturer, would produce wafers containing hundreds of unconnected gates (NOR, NAND, flip-flops). The designer (in this case, Sinclair’s brilliant engineer Richard Altwasser) decided how to connect those gates. This is where the ZX Spectrum ULA shines

Why use a ULA?

However, the ULA was a double-edged sword. It was uncommitted. Once you printed the mask, you couldn't change it. The Spectrum’s infamous "attribute clash" (color fringing) wasn't a bug; it was a physical limitation of how many gates Altwasser could fit into the die to decode video memory. However, the ULA was a double-edged sword

Early microcomputers had chaotic I/O. The Spectrum uses a "partial decode" strategy. To save transistors in the ULA, the Spectrum doesn't check all 16 address lines for I/O. Usually, it checks only the lower 8. flip-flops). The designer (in this case

In a normal microcomputer (like the Apple II), these tasks are split across separate chips. In the Spectrum, the ULA ate them all:

The Z80 CPU is paired with 16kb or 48kb of dynamic RAM (DRAM). Unlike static RAM, DRAM forgets data unless every row is read every 4ms (the refresh cycle). The Z80 has a built-in refresh register, but it’s weak. The ULA solution: The ULA steals bus cycles from the Z80. It pretends to be the bus master, fakes a memory read to refresh a row of DRAM, then hands the bus back. The Z80 never notices.