The ULA is the bus master. The CPU is the guest. Part 5: The "ULA Failure" – Designing for Reliability Ironically, the very chip that made the Spectrum cheap also destroyed its reliability.
Why? Because one engineer, armed with a logic analyzer and a Ferranti databook, looked at the problem of building a color computer for the working class and said: "I don't need a million transistors. I need 1,000 gates, configured perfectly." The ULA is the bus master
The ZX80 and ZX81 used discrete logic to generate video. The Spectrum needed color, but adding more chips would kill the budget. The solution was the —specifically the Ferranti ULA. The ULA is the bus master