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

However, the ULA was a double-edged sword. It was . 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.

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 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. However, the ULA was a double-edged sword

No is complete without expansion. The Spectrum’s edge connector gives direct access to the Z80 bus. But crucially, it also exposes the ULA’s control lines. The Z80 has a built-in refresh register, but it’s weak