ring crossing:

This article, last of the original three done altogether in 1990, on getting the critical pieces functioning independantly that we needed to do the port. Once these we obtained, the kernel was inevitable.

Booting a kernel didn't require all of this - but by extending support, we had a "mini kernel" like functionality. Dillemma - how much do you let the boostrap/loader actually do? We chose after the kernel was up to have it to the most - but this answer is subject to review.

We initialized the processor with initial descriptor and page tables - one needs to run with the tables before activating memory/interrupt kernel functions.

Here's an oddity we had in the initial port. We first avoided the 386 TSS context switch because it was slow. With the 486 and later, it wasn't so we used it. But we needed at lead one to transit rings.

System calls were handled by a call gate, a 386 peculiar mechanism which allowed for ring crossing into supervisor mode using a special segment descriptor call. We simulated user mode to test prior to running in the kernel.