Interrupts are something the calculator generates at a fixed rate. The normal speed is about 110Hz, or 110 times every second.
FnOff will turn these off. This gives you slightly faster speed, and you can do some stuff you normally couldn't because the OS would mess with it (Like linking). However, you cannot use the normal getKey, but you instead have to use getKey(9) et al. (Well, if you use fnInt( you can't. But otherwise it would just turn them back on anyway) FnOn turns these back on.
fnInt( can be used to make your own interrupt. The normal interrupt is the OS's and does stuff like check for keys, link activity, etc. To use your own interrupt, you use fnInt(LB,F), where LB is the subroutine you want to execute on the interrupt and F is how often to execute it. It can be 0, 2, 4, or 6 and must be a constant, not a variable.
Say you had a subroutine I. To make it an interrupt that executes as often as possible, you can use fnInt(I,0 to set it up.
The routine I will execute at 110Hz. Note that you shouldn't use DispGraph or either of its modified versions in an interrupt.
In addition, DispGraph^rr might need an FnOn after it because it disable interrupts.

In assemblers like Brass and Spasm, you can use specially named labels for local purposes.
For example, in Brass syntax, your above loop could be
-
    dec c
    jr nz,{-}And you can use it again later in a similar manner, with no naming conflicts. Note that Spasm's syntax is different.
Depending on what you're doing, this could also be a good solution. (You will have to upgrade from TASM, though )
Edit: Note that in Brass, you aren't required to put colons after your labels in most circumstances.

The answer to life, the universe, and everything

I think he means, instead of having greyscale, have it in black and white but with a texture that approximates the grey.

Only Ndless recognizes it?
Windows should at least acknowledge that it is connected... Do you get any notification, any change in the tray, when you plug in or remove the calc?

The OP "registers" are actually six 9-byte (?) memory areas in RAM, and are specific to the TI-OS.
There will probably be an alternate strategy used

Sounds good.
program4: The solution to that is to package the library with the program, and perhaps give the device a central libraries location. If the libraries the program needs are already there, you can just have the device discard the extra copies.