physical memory:

Dealing with PC reverse compatibility requires defeating logic like Gate A20, which forces address rollover to a single megabyte for ancient 8086/8088 compatibility.

This, the first article, is the first published mention of 386BSD. By this time, the project had been operational for 18 months, and William Jolitz was at Berkeley working on the Net/2 release.
In this installment, we discussed the beginning of our project and the initial framework that guided our efforts, in particular, the development of the 386BSD specification.

Most popular microprocessors use either segmentation or paging to manage memory address space access. The 386 is rare in that it possesses both. In fact, since segmentation, (see Figure 3(a)), is placed on top of paging (see Figure 3(b)), you are expected to use segmentation in some form any time memory is paged. And, most important, BSD relies on paging.

A UNIX legacy, the "u." or per-process data structure, which held the kernel-related data of a process, was present on 386BSD prior to February 1991.

The 386 Paging Memory Management allocates memory in 4KB and 4MB allocation units. This impacts the way programs execute and share file data.

The actual memory referenced on processor, system, and I/O device busses, or physical memory addresses have standard assignments for the kernel to manage for the operating system to function.

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

Whats different about operating systems like Unix that use a root filesystem, and other systems that don't require a filesystem to be mounted initially to operate?