Guide to the TD4 4-bit DIY CPU
This detailed guide chronicles the hands-on journey of assembling and understanding a 4-bit DIY CPU kit, the TD4. The author meticulously explains its limited but instructive architecture, from soldering challenges to demystifying its instruction set and internal logic. It's a perfect deep dive for hardware enthusiasts and those curious about the foundational principles of computing.
The Lowdown
The author embarks on an educational project, building a compact 4-bit CPU kit known as the TD4, which features just two registers, LEDs, and 16 bytes of program ROM. Dissatisfied with the kit's sparse documentation, they've created a comprehensive guide to help others navigate the complexities and truly grasp the underlying computer architecture.
- Assembly Challenges: The article details the practicalities of building the kit, highlighting difficulties encountered during soldering, such as correctly orienting surface-mount diodes and attaching the USB connector (used solely for power).
- Architectural Overview: It provides a high-level explanation of how the TD4 operates, describing its DIP switch-based program ROM for instructions like ADD, MOV, IN, OUT, JNC, and JMP, and how these control the addition unit and register latches.
- Deep Dive into Components: The guide delves into the specific integrated circuits (ICs) and their functions, including the address decoder (demultiplexer), command decoder (combinatorial logic), data selector, adder, and register operations. It also explains the crucial role of the carry flip-flop for conditional jumps.
- Programming Examples: Practical programming demonstrations are offered, ranging from simple LED output and flashing programs to more complex tasks like counting up, down, and then back up. These examples illuminate the constraints of the 4-bit system and the nuances of its modular arithmetic.
- Development Tools: The author, along with a collaborator, developed a rudimentary Python-based assembler and simulator for the TD4, showcasing a practical approach to working with such a limited instruction set.
- Further Exploration: The post concludes by drawing parallels to other educational computing projects like Nand2Tetris and Ben Eater's computer, while also pondering advanced topics such as model checking, program synthesis, and the historical context of 4-bit computing.
Ultimately, this guide serves as an invaluable resource for anyone looking to build and intimately understand the workings of a minimalist CPU, proving that even a simple 4-bit machine can offer profound insights into computer science fundamentals.