The way most programs/apps  that do that use a font hook instead of modifying a Flash location. Overwriting the font data would be a pain (backup parts of the page that should not be overwritten, then erasing the page containing the data, restoring the backup content, and writing to the proper parts).

Hey, so I was writing this post when I realized that I can make fonts work on the 83+BE.

First of all, we need to remember that the font hook needs to be lightning fast. Basically, it has to be so fast that people don't even realize it's running. Here's why: the font hook is called for every single character that is displayed to the screen. For simple tasks like typing, only 1-5 characters are displayed at at time, but when you press clear, open a menu, or scroll a menu, up to 128 characters have to be rendered. At 100 characters per render, each .001 second that my font hook takes corresponds to 1 extra second to scroll 10 names in the program menu. (Which is enough to make you turn the hook off)

Next, we need to look at the memory setup I have to deal with. zStart is an app, which means that it is going to be executing from a flash page. We'll assume the standard situation where the font is archived so it is also on a flash page. Lastly, the location where I need to copy the font data is in the $845A region. Also, to make things worse, we're running in a really common hook, which means we can't touch any memory besides $845A which is what we're supposed to change.

So, in order to get the job done, we need to have zStart executing code, the archived font, and $845A all exposed at the same time. We have three memory blocks, $4000, $8000, and $C000. On the 84+, $4000 can have either flash or ram, $8000 can have either flash or ram, and $C000 can have any of the ram pages. Here's the setup I use to do the copying:
$4000 - zStart
$8000 - flash page of font
$C000 - ram page 1 (this allows me to virtually move $845A to $C45A)
In this position, the copying is rather easy, just stream from $8000 to $C000.

There is another way I could have done this to avoid the memory mess, and that is:
$4000 - flash page of font
$8000 - regular with zStart code copied here
$C000 - flash page 0 (this won't be used)
This works, but there are two key flaws with it: 1) This is a very common hook, so if I destroy like 200 bytes several times a second, I'm going to break a lot of stuff 2) Copying 200 bytes every character would severely slow the calculator down


So that's the reason I didn't think I could do it before. But I just figured out how I can do it. Instead of copying the font data to $845A immediately, I'll copy it into a part of the stack that I allocated, and then I can copy that back to $845A.

Next time I work on zStart, this is going to happen.