The TI-84 Plus just had its operating system fully reverse-engineered and posted online.
A developer published a complete reverse-engineering of the TI-84 Plus OS. The calculator runs on a Zilog Z80 processor - an 8-bit architecture dating to 1976 - and Texas Instruments has never shipped the OS source code publicly. Prior community efforts have mapped pieces of the device's memory layout and ROM calls, but a full annotated teardown goes further than any previous public work.
The TI-84 Plus is one of the few pieces of consumer electronics that institutions still mandate by name - it sits on the approved device list for the SAT and ACT. That captive market has let TI charge over $100 for hardware built around an architecture that was already aging at the calculator's launch. A complete OS map lowers the barrier for homebrew software and gives researchers a clearer picture of what is, in practice, a locked-down computing environment used by millions of students annually.
Texas Instruments has historically tolerated the calculator hacking community more than it has litigated it - the community's enthusiasm keeps decade-old hardware culturally relevant. Whether that tolerance extends to a fully annotated OS publication is a different question.