The Zx Spectrum Ula How To Design A Microcomputer Pdf 57l

The ULA’s most famous "flaw" is attribute clash – where changing the colour of one character cell changes the colour of the entire 8x8 block. The PDF will show why: The ULA has only one address bus for fetching screen data. It fetches pixels (bitmap) during one scanline and attributes during the next. By studying this limitation, you learn a vital lesson in memory bandwidth management for your own design.

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Before FPGAs and ASICs, there was the Uncommitted Logic Array. Ferranti’s ULA was a gate array: a silicon wafer pre-populated with unconnected NAND gates, NOR gates, and flip-flops. The final "wiring" (the metalization layer) was custom-designed by the customer—in this case, Sinclair Research. The Zx Spectrum Ula How To Design A Microcomputer Pdf 57l

The ULA did not run software. It was hardware. Specifically, in the ZX Spectrum, the ULA was responsible for:

Why "How to Design a Microcomputer"? The Spectrum had only two major chips: the Z80A CPU and the ULA. Everything else (ROM, RAM, passive components) was support. Designing a microcomputer using a ULA meant you didn't need to wire up 50 separate logic chips. You defined the logic in a schematic, sent it to Ferranti, and six weeks later you had a single custom chip. The ULA’s most famous "flaw" is attribute clash

1. The DRAM Controller Logic The Spectrum uses 4164 (64kbit) DRAM chips. These require row and column addresses to be multiplexed. The ULA contains a simple state machine that:

2. The Contention Flip-Flop This is the most infamous circuit on page 57. It uses a few NOR gates to detect when the Z80 tries to access the RAM at the same time the ULA is reading video data. Instead of a proper bus arbiter, Sinclair used a simple flip-flop that halts the Z80’s clock for half a cycle. Before FPGAs and ASICs, there was the Uncommitted

3. The Clock Divider The Spectrum uses a 14.218MHz master clock (derived from the PAL colour burst frequency). The ULA divides this down to 3.5469MHz for the Z80. Page 57 would show a binary counter chain creating the 4T and 8T cycles.