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

Your computer needs a monitor and/or BASIC. Options:

The Spectrum’s genius is its low chip count: Your computer needs a monitor and/or BASIC

Chris Smith’s work is highly regarded for its approach, which involves stripping the chip down to its circuit diagrams. Up to 40% of dies failed final test

Sinclair purchased "grade C" wafers (cheapest). Up to 40% of dies failed final test. However, because the ULA was so integrated, a single failed gate could brick the machine. Sinclair’s solution? Underclocking. A ULA that couldn't manage 3.5MHz might run at 3.4MHz. A ULA with a dead keyboard column might have that column disabled in the ROM. Underclocking

A ULA is a "semi-custom" chip. Ferranti would manufacture a base wafer with thousands of unconnected logic gates. A customer (like Sinclair) would then provide a single final metal layer to "wire" those gates into a specific circuit. This was the precursor to the modern and FPGA . Key functions of the ZX Spectrum ULA included: