uCR is not a kernel, or a micro-kernel. There are no system calls and there are no task structures such as page translation tables or system call gates. Calls to uCR operations are ordinary function (or method) calls.
uCR also is not intended to abstract the board away. Operating systems that try to abstract the hardware away wind up instead requiring that the hardware be a certain way. That is frequently counter-productive. The uCR core does nothing to devices other than the CPU, and does nothing to get in between devices and the programmer.