user mode:

386BSD ignored as much as possible the segmentation hardware of the x86, preferring to use a "flat" address space.

Catching terminal processor faults.

Sometimes you're forced to use processor features - like hardware context switching. Origionally, the earliest versions of 386BSD didn't use the hardware context switch TSS feature - but you still had to have one anyways.

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.

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.