A:

It all depends on the platform:

On Win32, Win64 and PS3, callbacks are usually made from a background thread.

On MacOS, callbacks are usually made from an interrupt-based system callback on MacOS 9 and a background thread on MacOS X.

On PS2, callbacks are usually made from a hardware interrupt.

On Wii, callbacks are usually made from a hardware alarm interrupt.

On DOS, callbacks are usually made from a hardware interrupt.