I know one of the major issues with writing programs with more direct access to the calculator's hardware is the limitations put in place by TI-OS, and flashing a custom OS is blocked by the boot code which checks for a signature from TI. The section of flash memory that contains this code is write protected. So to be able to add a custom OS to the calculator (a permanent one at least) one would likely have to replace the flash chip entirely with one that does not contain the signature check code. I've looked around online but have been unable to find the exact flash chip (or set of compatible chips) that the TI-84+CE uses, and I'm not exactly keen on taking mine apart to find out, I still need it for school. Does anyone know?

P.S. I know that I wouldn't be able to do this myself, I don't have the programming knowledge nor the soldering skill for it. This is simply for curiosity's sake. If I got anything wrong please let me know as well.
The pre-revision-M chip is https://www.winbond.com/resource-files/w29gl032c_revh%20v3.0.pdf
It's the same chip used in the monochrome calcs, so you can use the commands on this page: http://wikiti.brandonw.net/index.php?title=83Plus:OS:Raw_Flash_Commands

I'm not sure what chip the Revision M / 83PCE EP uses, but it's very different.

The flash chip isn't super secure, and there are at least two different ways to overwrite parts of the boot code on pre-rev-M calcs. There's really not much reason to do so, though, as most OS tweaks can be done with hooks. You can "directly access" most of the calc's hardware even with the OS in place while running a program.

EDIT: since I didn't have the revM datasheet bookmarked, here it is: https://www.winbond.com/resource-files/w25q32jv%20spi%20revc%2008302016.pdf
and the wikiti page: http://wikiti.brandonw.net/index.php?title=84PCE:OS:Serial_Flash_Commands
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