![]() ![]() Unfortunately, on August 18th, 2016, Texas Instruments released the OS 4.3 version, which blocked the ability to install Ndless 4.2 and additionally forbade going back to OS 4.2. Ndless is the TI-Nspire's "jailbreak", which makes it possible to run programs much more powerful than those allowed by TI's approved tools (Basic, Lua), by taking advantage of native code (C/C++/ASM/etc.). But when you know that since OS 5.1.0 you can easily crash your calulator with only 3 short lines of TI-Basic, it's still being harmful to all TI-84 Plus CE owners. All OSes 5.1.5 to 5.2.2 have a minimal allowed version set to 5.1.5, which might appear as non-excessive to you. The OS installation code from previous 5.0.0 Boot Code versions doesn't seem to care about the new minimal allowed version TI-Certificate field. Apparently, the downgrade protection is only functional on TI-84 Plus CE coming preloaded with Boot Code 5.1.5. In the later, the transfer is immediately aborted. ![]() So the calculator immediately checks if the received version is higher or lower than the minimal allowed OS version. When you start an OS transfer, the TI-Certificate being at the start of the file is among the first sent things. The TI-Certificate following the header of any 5.1.5+ OS file is now holding 2 versions number : - the OS file version - the minimal allowed OS version to set When an OS install suceeds, the minimal allowed OS version, if higher, is written to the calculator Flash memory, in the TI-Certificates page, a zone which you can't reset with official menus. In fact, it is very similar with the TI-Nspire downgrade protection, except that just the first 3 version digits are being checked. For the first time in history, we now get a downgrade protection on a TI-8x Flash calculator, the TI-84 Plus CE. They wanted to prevent us from being able to install modified OSes, for example with a patched exam mode. But the intent wasn't to prevent downgrading, or they would simply have checked the version. With the only exception of TI-84 Plus coming preloaded with Boot 1.03, in which TI had moved from a 512-bits RSA signature to a 2048-bits one - so the only accepted OS were 2.55+ (so 2.55 only, as no update has been released since). You could update and then downgrade freely. Owning a TI-8x Flash calculator had always been a synonym for freedom.
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