The filling routine is essentially the same, I might have tweaked a few numbers here and there, but the bulk of it is the same. 
Pruning down the amount of DispGraphs made a HUGE difference.

About the undo button: I've decided that if it's used, you can't record your time for that particular minefield. 
So there's a penalty for using the Undo button - you can't really 'win' the game if you use it.
I just added it for those users who are less concerned about earning a high score more concerned about being able to actually finish the game.
(If I ever distribute this to my classmates, there's a high that they don't know how to play minesweeper.  This might soften the learning curve for them.  Maybe.)
Maybe I should add an option to disable showing the location of all the mines when you lose?

Aah I see, so it was definitively the DispGraph making such a difference. Now you know how slow the TI-82/83/84/+/SE LCD drivers are .

As for the Undo, maybe add difficulty modes or bonuses for when someone wants to disable it. Another option to disable showing the location of mines when you lose and a bonus for it seems like another cool idea.

Aah I see, so it was definitively the DispGraph making such a difference. Now you know how slow the TI-82/83/84/+/SE LCD drivers are .

As for the Undo, maybe add difficulty modes or bonuses for when someone wants to disable it. Another option to disable showing the location of mines when you lose and a bonus for it seems like another cool idea.

I don't really like the idea of using bonuses to reward the player if they chose to disable the Undo button.  If the player avoids clicking the undo button, there's really no difference between the player who leaves it enabled and the one who disables it.

I think that instead of rewarding the player, I should penalize the player for clicking the undo button - in exchange for having unlimited shots at solving a particular minefield, they can't save their time to the high scores list.

Besides, I can't really think of any kind of bonus I could give.

But I think I will add an option to toggle to show the location of the mines when you lose.

Ah, right. I guess penalizing might be better.

Sorry for not posting an update for so long, but I decided to rewrite my code, which took awhile.
Well, the rewriting was easy, but the program was plagued by many inexplicable bugs.  Now, several coding sprees and many blasphemies later, I now have a program that is bug-free (as far as I can tell), and actually includes several new features (pause and a better options menu). 

I've rewrote the code so that it'll be easy to integrate scrolling and high scores, plus the entire code feels much more streamlined and less patchy.
I'll do that later; I'm pretty tired right now.
Meh, I didn't expect to be debugging this long, it's not like the rewrite was drastic, or anything...

I think I'll get scrolling, high scores, and the timer up by Saturday at the very latest.

That looks really cool! Good luck with the rest of it

Do you think scrolling will slow the grayscale down?

Nah, probably not.  Or, at least, it won't be too noticeable.  It might be slower when a blank bubble is expanding (but only because there are so many more squares to check).
The way I'm envisioning it, integrating scrolling should only add about 5-10 lines max, as long as they're carefully crafted and written.

Right now, my greatest concern is getting the timer and the getKeys to work together nicely. 
I seem to have an irrational phobia of interrupts - it's probably because my calculator keeps getting RAM clears almost every time I try experimenting with them...

~~~~

Questions:
Is an 18x18 maximum field fine?  I think that if I use appvars instead of L1 to store my map data, I could increase the size dramatically at the cost of taking up RAM.  (A 40x40 field would take up about 3200 bytes, for example.)
When compiled, the program is 7873 bytes.  Is this a reasonable size?

I don't know if you have room for this idea, since I don't know how much room your tiles take, but when you add scrolling in if you could have like an arrow pointing in the ways you can move, like a small one, that could be helpful. It could tell you when you hit the end and such. Just a thought