A: One protection is both different...

I'm of course referring to the latest twist in the microkernel story: Singularity vs L4. The recent public release of the Singularity source by Microsoft has generated some attention, and invited some comparisons with L4. It has also “enriched” the blogosphere with blatantly incorrect statements such as “the use of SIPs effectively eliminates the overhead traditionally incurred by context-switching in conventional microkernels.”

So, what's going on?

L4 and Singularity can be considered as the leading microkernel representatives of two alternate approaches to protection: hardware or software mechanisms. The L4 approach is to use hardware mechanisms (processor modes and the MMU/MPU) to protect processes (and their data) from each other. This is the standard way for providing protection in operating systems, and the idea of microkernels based on this approach goes back to Brinch Hansen's Nucleus (1970).

The alternative, used by Singularity, is to use software means, specifically language-based protection (using the type system) and static analysis. While most people think this is a revolutionary idea, it is actually quite old. The approach has been used by the Burroughs mainframes since 1961! (I used a B6700 a lot in a former life, and it was sloooooow!)

There is no such thing as a free lunch, and protection is no exception.

The cost of hardware-based protection comes in the form of . . .

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