![]() ![]() 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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