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

The ULA demands that DRAM refresh and CPU access occur in specific 4MHz clock phases. The Z80 CPU (running at 3.5MHz) must be halted (via the /WAIT pin) during the ULA’s screen drawing phases. This is the famous contention .

The 324-page volume is structured to take a reader from basic semiconductor theory to full system implementation: www.librador.com The ZX Spectrum Ula: How to Design a Microcomputer - Amazon

To the naive observer, slowing the CPU down 30% to draw the screen is a flaw. To the systems engineer, it is genius.

Software support came from Steve Vickers (of Nine Tiles Ltd), who adapted Sinclair BASIC from the ZX81 and wrote the Spectrum’s ROM firmware. Together, Altwasser and Vickers created a machine that was both affordable and capable, attracting a generation of programmers and gamers. The ULA demands that DRAM refresh and CPU

If the CPU tries to access the lower 16KB of RAM during this time, the ULA pauses the CPU by pulling the low.

The Zilog Z80 CPU and the ULA both needed access to the same 16K bank of Dynamic RAM (DRAM). The ULA needed constant access to draw the screen smoothly, while the CPU needed access to run game code.

The ZX Spectrum ULA (specifically the 5C112E and later 6C001E variants) acted as the traffic cop, video generator, and audio manager of the entire computer. It directly handled four primary system architectures: 1. Video Generation and the Color Artifact The 324-page volume is structured to take a

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Because the magic of the Spectrum wasn't despite the ULA—it was because of it.

Designing a retro computer like the ZX Spectrum means mastering the centralized timing and I/O logic that the ULA once held. Don’t simply copy the Spectrum – improve it. Remove contention by adding dedicated video RAM. Add sprites. Use modern SRAM. But always respect the core lesson: Together, Altwasser and Vickers created a machine that

The Spectrum ULA maps the keyboard to port 0xFE . It reads 5 rows of keys (Shift, Z–M, etc.) via IN instructions.

For the Spectrum, Sinclair’s mandate was absolute: The traditional solution (a dedicated Video Display Controller like the Motorola 6845) was too expensive and required external character generators and RAM. The ZX Spectrum ULA was the answer: a custom gate array designed by Richard Altwasser of Ferranti, programmed to do just enough and nothing more .