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toaruos
Discontinued A completely-from-scratch hobby operating system: bootloader, kernel, drivers, C library, and userspace including a composited graphical UI, dynamic linker, syntax-highlighting text editor, network stack, etc.
> [..] in kernel mode while retaining the ability to compare how the very same usermode bits run on the Linux kernel as well. That's a unique feature in the realm of educational kernels.
There's also Kerla: https://github.com/nuta/kerla
Somewhat related, Managarm[0] also attempts to be Linux compatible, but has a microkernel, multiserver design.
0. https://github.com/managarm/managarm
Unless you want really high performance networking, you don't need tons of RAM to implement it. ESP32-based MCUs are known to implement Wifi + TCP/IP + TLS [1] on 1-2MB of RAM. This is, of course, without a Linux-compatible kernel.
[1]: https://github.com/espressif/esp-idf/tree/master/examples/pr...
Another interesting project to checkout is klange’s toaruos https://github.com/klange/toaruos
So it used to be possible to compile a (lightly patched?) Linux with tcc, which was part of the really cool "live disk that compiles linux and then boots it in a matter of seconds" demo[0]. I think the problem is that kernels (need to!) care about extremely precise implementation details that most programs don't need; things like "this data structure of exactly this many bytes must be placed at this exact memory address with this exact alignment, then we need to set the CPU registers to this exact list without touching the stack, then run this CPU instruction to make the hardware do the action we just staged", and they care about doing so with good performance, and AIUI, spec-based C either can't do all the things that modern OSs want, need you to jump through a lot of unergonomic hoops, or don't get the performance that people want. Hence, compiler extensions to do what the kernel wants to do while minimizing undefined behavior but keeping performance. Honestly, the fact that every major OS (I specifically know about Linux, every BSD, illumos nee Open/Solaris, but I'd be shocked if NT/Darwin were different) needs to extend the compiler is probably a glaring criticism of the C standard.
[0] https://github.com/seyko2/tccboot