Designing a modern microcomputer based on the ZX Spectrum requires a deep understanding of its custom "heart": the Ferranti Uncommitted Logic Array (ULA)
. This chip combined disparate logic functions—video generation, keyboard scanning, and memory management—into a single, cost-effective package. Amazon.com The Role of the ULA in ZX Design The ULA was the bridge between the
and the rest of the system. In any retro or portable recreation, you must account for its primary responsibilities: Video Generation: It reads the lower 6.75 KB of RAM (Video RAM) to generate a pixel display. Memory Contention:
To avoid hardware conflicts, the ULA has priority access to "contended memory" (the first 16 KB of RAM). It can physically stop the Z80's clock to take control of the bus. I/O Management: It handles Port , which manages the border color, beeper sound, and the keyboard matrix. Universidad de Valladolid How to Design Your Own Microcomputer
Whether building a desktop replica or a portable "Speccy," there are three main hardware paths: Discrete Logic (The "Harlequin" Method): Projects like the Harlequin board available on replace the unobtainable ULA with standard TTL logic chips
. This is the most educational approach for understanding digital circuits. FPGA Implementation (Modern & Flexible): Most modern clones, such as the ZX Spectrum Next Field-Programmable Gate Array (FPGA) to mimic the ULA's logic. Intel Quartus Prime or Xilinx Vivado to program the hardware logic in Designing a modern microcomputer based on the ZX
FPGAs allow for enhancements like VGA/HDMI output, SD card storage for instant game loading, and even "ULAPlus" for expanded color palettes. No-ULA Design: Some hobbyists, featured on
to handle the logic usually reserved for the ULA, simplifying the build for those who want to use a real Z80 but avoid complex gate arrays. Portable Design Considerations
To make your microcomputer portable, you'll need to integrate:
The ZX Spectrum ULA: Designing a Modern Retro Microcomputer The Sinclair ZX Spectrum remains a masterclass in minimalist engineering. At its heart lies the Uncommitted Logic Array (ULA), a custom chip that consolidated dozens of standard components into a single piece of silicon. Understanding the ULA is essential for any enthusiast looking to design a retro microcomputer or a portable modern recreation. The Heart of the Machine: The Ferranti ULA
In the early 1980s, Richard Altwasser and the Sinclair team faced a challenge: build a color computer for under £100. Their solution was the Ferranti ULA, a "gate array" precursor to modern FPGAs. The ULA performed four critical roles: Video Generation: It read screen memory (a linear
Video Generation: It handled pixel and attribute (color) data from memory to generate a PAL video signal.
Memory Management: It managed "contention," pausing the Z80 CPU when both needed access to the same bank of RAM.
I/O Control: It scanned the rubber membrane keyboard and managed audio input/output for cassette tapes.
Clock Timing: It divided a 14 MHz master clock down to 3.5 MHz for the Z80 CPU. How to Design Your Own Retro Microcomputer
Designing a modern "Speccy" clone or a unique 8-bit machine involves translating these 40-year-old logic circuits into modern components. 1. Logic Implementation You can recreate the ULA's functions using several methods: 0xFE for keyboard
The ULA was the "glue logic" that made the Z80 CPU work with the rest of the system. Its core jobs were:
Key lesson: The ULA was a tightly coupled video+memory controller. You cannot make a simple ZX clone without replicating this arbitration.
So, how does this history lesson help you build a "portable retro computer"?
If you are building a ZX Spectrum portable (often called a "ZX-clone" or handheld), you have two paths:
.tap or .z80 files directly.You can strip down an original Spectrum board. However, the ULA presents a challenge: it outputs 15KHz video, which modern LCD screens (usually accepting HDMI or VGA) cannot display without a scaler.