Are microkernels hardware abstraction layers?

In a recent post Gernot made a comparison between nanokernels and hardware abstraction layers (HALs). This prompted a question on the OKL4 developers mailing list: well, couldn’t you consider a microkernel a HAL?.

I think the logical conclusion, both theoretical and practical, is a resounding, no.

Why? Well, a microkernel is, in theory (if not always in practise) minimal. That is, it should only include things in the kernel are those pieces of code that must run in privileged mode.

So, if a microkernel was to provide any hardware abstractions, it will only be providing the abstraction that have to be in the kernel. Which really falls short of a complete hardware abstraction layer.

Now, probably the more interesting question is should the microkernel provide any hardware abstraction, and if so what hardware should it be abstracting, and what is the right abstraction. After starting to write some answers to these questions I reminded myself of the complexity involved in answering them, so I will leave these questions hanging for another post.