I'm reading through the MIT xv6 OS handbook and code (here: https://github.com/mit-pdos/xv6-riscv/) and they mention the fact that they created it as a monolithic kernel since most unix systems are monolithic. They then introduce the microkernel concept. Are there microkernel concepts out there (especially code) I can check out? I'm curious to see how userspace processes communicate to kernel processes to execute privileged actions.