BIOS:

First thing the kernel does is open its root filesystem - so we need a way to write a filesystem onto the hard disk, adjacent to the DOS filesystem, which is what this utility does.

ISA devices that attach to the AT standard through controllers field a broad number of devices various categories.

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.

We stepwise proved out the running environment of program tools, program loading and execution, trap handling, stack, and support of high level language. Our project moved from fragile to substantive.

Since we had a program running in an empty PC, we needed primative input with a keyboard driver for polled input. Used by standalone programs to boot and test parts of the kernel.

Output from our standalone programs was done by a primative display driver to allow standalone programs to display results from bootstrap and test programs. We avoided the BIOS entirely.

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?