[Marius Taciuc’s] latest endeavor, the B4 Thinker, offers a captivating glimpse into microcontroller architecture through a modular approach. This proof-of-concept project is meticulously documented, with a detailed, step-by-step guide to each component and its function.

Launched in 2014, the B4 Thinker project began with the ambitious goal of building a microcontroller from scratch. The resulting design features a modular CPU architecture, including a base motherboard that can be expanded with various functional modules, such as an 8-LED port card. This setup enables practical experimentation, such as writing simple assembly programs to control dynamic light patterns. Each instruction within this system requires four clock pulses to execute, and the modular design allows for ongoing development and troubleshooting.

While still in its draft phase with some incomplete components, the B4 Thinker project stands out for its educational value and the thorough documentation provided. This modular approach not only supports incremental improvements but also serves as a valuable learning resource for those interested in computer architecture and design.

It seems like four bits is the sweet spot of not to complicated, but not too simple. So we see a lot of 4-bit CPUs. Some even use discrete transistors.