Is the 'HAL dll' the lowest level file?

The HAL (Hardware Abstraction Layer) has a dll in windows called 'hal.dll'. This is even below the kernel level. It's the lowest level file on a windows box isn't it?

Is it even below the 'hardware bus driver'?



... Another low-level thing is 'IRQL levels'.
IRQL is like a hierarchy within ring-0.

Just how low-level can things get?
i.e (When do you hit the absolute basement)?