According to OpenNet, developer Dmitry Grinberg managed to run the Linux kernel with a rootfs environment from Debian on a 10-micron 4-bit Intel 4004 processor, released at the end of 1971 and considered the world’s first commercially available single-chip microprocessor. The Intel 4004 has only 2300 transistors. The processor had only 46 instructions, and its peak performance reached approximately 93 thousand operations per second.
Due to the impossibility of directly porting the kernel to the Intel 4004 and due to the limitations of the CPU itself, the author decided to write an emulator for the MIPS R3000 processor, within which Linux was already running. To run the processor, the author created a Linux/4004 board using several approaches based on components from the 1970s, which, as it turned out, were not so cheap. Naturally, the board also contains much more modern components that allow, for example, the use of an SD card as permanent memory.
Image source: dmitry.gr
Due to the low performance of the Intel 4004, the emulator worked slowly – it took almost 4 hours of real time to process each virtual second in the emulated environment. With improvements to the board and software, the Linux boot time dropped from almost 9 days to about 5 days. The author was even able to overclock the CPU from the base 740 kHz to 790 kHz. Those wishing to repeat the experiment can use the published specifications and board diagram, as well as software.