What is the Smallest CPU for Unix?

When we delve into the history of Unix and its implementation on various CPUs, the question of the smallest bitwise CPU that Unix has ever run on becomes intriguing. Historically, the PDP-11 and Z-80 are often highlighted as notable platforms for Unix implementations, each bringing unique capabilities and limitations to the table.

The PDP-11: A Widely Used Platform

The PDP-11, a 16-bit minicomputer developed by Digital Equipment Corporation (DEC) in the early 1970s, is the most recognized platform for early Unix implementations. Although Unix was originally developed on the PDP-7, a 18-bit machine, the PDP-11 played a crucial role in popularizing Unix. The PDP-11 could address up to 4 MB, significantly more than the 64 KB original 16-bit limit.

Technically, different versions of the PDP-11 could have memory address busses ranging from 16 to 22 bits, supporting larger memory capacities. For example, the PDP-11/40 could address up to 1 MB, while the PDP-11/70 could address up to 4 MB, showcasing the adaptability of the platform in running Unix.

The Z-80's Capabilities

The Z-80, an 8-bit CPU, is another interesting case. Despite being an 8-bit CPU, the Z-80 could manipulate 16-bit data and address significantly more memory than the 8-bit limitation would suggest. With external memory support, the Z-80 addressed over 64 KB, which was quite impressive for its time. Unix clones have indeed been written for the Z-80, demonstrating its capacity to run Unix-like systems, albeit with some limitations.

Similarly, the Intel 8088, an 8-bit data bus version of the 8086, had both an 8-bit internal data bus and a 16-bit internal address bus. This system addressed up to 1 MB of memory, exhibiting the flexibility of these early architectures in accommodating Unix.

The Question of Memory Footprint

While the smallest CPU for Unix from a bitwidth perspective is often considered to be the 8-bit Z-80, another perspective could shed more light on the topic. A more intriguing question might be: What is the smallest version of Unix ever implemented in terms of memory footprint? In the early days of Unix, Bell Labs attempted to create a minimized version of the Unix kernel, reduced from around 50 KB to as low as 24 KB, known as mini-UNIX. This version ran on a PDP-11 without memory protection, with a total of 56 KB, leaving 32 KB for user programs.

Furthermore, there was another effort to strip down Unix into just 16 KB, running on a 40 KB LSI-11, which was a single-user-only implementation. These mini-versions of Unix were derived from the real PDP-11 Unix C source code, indicating the adaptability and portability of Unix across different platforms.

Conclusion

The history of Unix on different CPUs is a testament to the adaptability and growing capabilities of these systems. The PDP-11 and Z-80, despite their different bitwidths, were capable of running Unix, showcasing the evolving nature of computing in the 1970s and 1980s. The efforts to minimize Unix to very small memory footprints further highlight the journey of Unix from its origins on larger machines to its adaptability on smaller platforms.

For those interested in the technical and historical aspects of Unix, exploring the Bell Labs mini-Unix projects can provide valuable insights into the challenges and solutions of miniaturizing Unix for various computing environments.