bootstrap:

The second article in the "PORTING UNIX TO THE 386" series discussed the utilities we had to build to test the port on an actual 80386 PC.
By far, the most popular article.

Its not enough just to load the kernel and go, you need to insure its the kernel bit-for-bit by the time you run it.

After loading the kernel program, we enter protected mode to start up the 386BSD kernel, itself a protected mode program like its own applications.

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.

The 386BSD specification was in two parts - one that detailed getting to a operational system that could build itself and basic console applications, and a more extensive community involvement part, called "A Modest Proposal".

Choosing how far to go in supporting X86 architecture in order to get a basic BSD UNIX system to be able to bootstrap the futre efforts.

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.

How to bootstrap the system from hardware, loading the kernel program, itself a protected mode executable from secondary / nonvolatile / disk storage.

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.

Originally the test framework to prove kernel parts work in the environment of an protected mode program running on the "raw" machine, this became the bootstrap environment to later load and start the kernel in system operation.

We describe the need and use of a cross-support environment to create 386 code from a non-386 machine, so as to create the initial binarys before our port can generate them.

We used another proven UNIX system - a Symmtric 375, to cross compile 386 code (bootstrap, kernel, shell, utilities) to get the first system running.

Our use of cross-tools was simply to bootstrap onto the 386 - not to perfect ongoing development. So, if we found a problem with the tool, we bypassed it with a one time workaround - cross tools didn't need to work for everything.

We build the first instance of the root filesystem - before any operational system is present on the 386 to build one. Part of the bootstrapping cycle of getting up the first running system on a new architecture.

Portions of the root allow the kernel to be installed as the fundamental component of the operating system. These standalone programs occupy space within the root filesystem.

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?