Is the 'HAL dll' the lowest level file?

[Mkz]

IMHO the HAL is the lowest level you get. Lowest in the sense that it doesn't rely on any layer to access the "important stuff" in the hardware, such as changing IRQL status, for instance.
If you get the intel manuals and look at the code in the hal, you'll see it using the reserved memory ranges mentioned, the interrupts, etc.

You can replace just the HAL to make it work on another "platform". Not a different cpu (with different opcodes), only a different architecture (such as multi-processor instead of uniprocessor) but with the same instruction set.