kernel timers and clocks

[deco33000 at yandex.com]

Hello,

I am trying to figure out how timers work for the amd64 platform on freebsd 10.

How the kernel manage the timers? is it using TSC?

I see lot of functions using getnanotime and friend but I don't get how they work. 
They just iterate over time and read a volatile value. Where/how is that value kept updated?

Thanks

-- 
Jog