regular operating systems vs small real time operating systems

[Beta What]

Hi all,

I come from electronics background, and know how RTOSes for small
microcontrollers/CPUs work. Everything is one C program, and the
"tasks" are just separate void returning functions (which can call
other functions) that are part of the whole program. RTOS's
functions, which are part of the same program, handle juggling of
function pointers and there is a funciton associated with hardware
timer interrupt that triggers the RTOS functions to run.


Now, how does a larger computer system work? The kernel is a C program
with a main function, but then every "process" is also a program with
their own main functions. So how is the kernel program calling other
main functions? In fact, how is it juggling all these separate C
programs each with their own main?

Can anyone elaborate on the difference between a RTOS task and a large
OS process?

Also, why do they call linux/unix/windows "multitasking" operating
systems, when the units of processings are processes not tasks (that
is, each process has a separate main function).

A link or document would be good too, or a reference to book(s).
Thanks.