what you're talking about used to be called a 'monitor' program. you might get some interesting info at osdev.org and taking a look at colorforth.
[afterthought] google Rick Hohensee's ramblings on comp.lang.forth also.
[much later afterthought] to really get to basics, get a plug-in circuit breadboard, some old EPROMs, a 5VDC power supply and a 25VDC power supply (24 ought to work, it's more common). Use pullup or pulldown resistors on the address and data lines, and switches to supply ground or power to the address and data lines. Step through the addresses using the switches, and enter the data as ones and zeros on the data bus, programming each with a pulse from the 25V. I've never actually done this manually, but I once wrote a program (http://jc.unternet.net/hacks/?1994-01-01-01) to do this from a Centronics port, and burned several programs that way. This was back in the day when Intel would send you free books for all their processors and auxiliary chips, complete with detailed instruction sets and timing.
When done, connect your processor to the EPROM, give it some juice, and watch what happens. Maybe make it strobe some LEDs or something before you start trying to integrate keyboards and monitors.
