Recently I came across an old recovery disk for my parents’ old 486 from 1993 (the first machine I really seriously programmed — the Tandy before it got some QBasic time, but nothing as crazy as a C compiler. Some real mode assembly though once or twice). I managed to cobble things together to get it booting in VirtualBox (you see, in those days booting from CD didn’t exist, so the CD itself isn’t bootable, it just has a bunch of zips that were written to floppies by an operational system — getting that up
Anyway, after it was up and running, I figured it might be fun to start tearing it apart (since the entire machine state fits in RAM (4MB RAM, 512MB harddrive, 527MB restore CD, a couple 1.4MB boot floppies) it’s easy to blow it away and restore it very rapidly.
So, let’s start from the beginning, the Master Boot Record. This the MBR laid down by DOS 6. Before this there’s the BIOS, but unfortunately I don’t have access to that (otherwise, it’d be a fun but much more complex piece of software to explore), and the MBR’s really tiny (just 512 bytes, of which only like 446 or so are executable code).