...than to click on a youtube video in the midst of having-written for well over an hour, without Lazarus(?) installed... :/

Whelp, suffice to say, I'm trying out @Shaos's instructions, here: https://hackaday.io/project/19160-pcjrcart/log/51792-creating-a-bios-extension/discussion-74094 and made some modifications to make it XT-compatible (test the current page, rather than assuming page 7). And some more modifications because my stupid video-setup doesn't show anything other than "|-_|| -| |_" when displayed in red, or most any color, for that matter.

Anyways, a great starting-point.

I wasn't planning on doing BIOS extensions, as the whole point (at this point) in the project, of doing a custom ROM, is to limit the number of executed-instructions to those I've actually (at that time, when I get there) implemented... Which, wouldn't at all be limited by the BIOS, itself, which is run before the BIOS extensions...

BUT: his work, there, is a *GREAT* starting-point... showing how to use nasm, and more...

And, I think, PCjr cartridges basically work *identically* to PC/XT bios-extensions. And some ideas 'round that link, regarding how to use those bios-extension/PCjr-cartridge memory-locations as Read/Write, despite not having a /WE pin... in which case you can throw custom hardware, there, too.